I'm a pretty big fan of Games Workshop, and I love the skaven armies. Here's my interpretation for them for my WhiteBox game.
Armor Class: 6 
HD: see below
Attacks: by weapon (+ special)
Clanrat: 1/15 XP
Globadier: 4/120 XP
Assassin: 5/240 XP
Plaguebearer: 2/35 XP
Plague Monk: 5/240 XP
Packmaster: 3/65 XP
Jezzail: 3/65 XP
Rat Ogre: 4/120 XP
Grey Seer: 10/1,400 XP
Stormvermin: 4/120 XP
Warpfire Throwers: 3/65 XP
Skaven are humanoid rats, nearly man-size in height. They walk on their hind legs and they use their hands as would a human. They are typically brown or black, though whites are seen. Their eyes are a deep red and reflect light, making them appear to glow at night. They are worshippers of a god called the Horned Rat. It appears that their goal is the complete domination of the land and the slavery of all other races. This includes orcs and goblins as well as humans and demihumans. As such, they have no allies among the other races.
All skaven have infravision to 60 feet. They speak Skaven, though some (20%) speak Common. Skaven also have the ability to speak with rats of any type, and often use rats as spies and forward lookouts. Skaven are constantly exposed to disease, and thus gain a +4 on their saves when exposed to some form of plague. Skaven are never attacked by rats of any type, though only packmasters can control rats.
Clanrats: The majority of skaven encountered are clanrats (~85%). They are usually encountered in packs of 3 to 30. They conform to the listed statistics, and have no special abilities. If clanrats are preparing for a major battle, they don chainmail and have an AC of 5 . They wield spears and short swords in battle.
Globadiers are skaven who know how to make poison wind globes. These globes are small crystal balls, which are hurled at the enemy. When the globes strike, they shatter, releasing a poison gas that covers a 10 foot area. Anyone who breathes the gas must make a saving throw or take 20 points of damage. Those who make their saves take 5 points of damage. One globe can be hurled per round, with a range of 10/20/30. If a blunt object strikes a globadier, he must make a saving throw or 1 to 3 globes will explode. Note that this will almost definitely kill anyone near him. Other skaven tend to give globadiers a wide berth during combat for this very reason. Globadiers wear gas masks, which grant them immunity to the poison gas. The mask only protects vs. globadier poison, other gaseous attacks are unaffected by it. Globadiers also carry a short sword for engaging in melee.
Assassins are similar to regular skaven except that they have 2 Hit Dice and have several special abilities. They are very stealthy, and can only be noticed on a successful Find Secret Doors roll if they have hidden. If they successfully attack someone who has not located them, they inflict double damage with their weapon. Assassins also gain a +1 on their initiative roll. They use contact poison on their blade (save or take 1d6 damage).
Plaguebearers go into battle wielding censers filled with a liquid that causes disease to beings which come into contact with it. Plaguebearers do not retreat, fighting to the death, as they are among the most insane skaven of the clans. Someone hit by a censer must make a saving throw or catch a disease, which has the following effects: victim loses one point of constitution and strength per day. If either ability reaches 0 the victim dies. The spell Cure Disease is the only known cure for the ailment. Once the spell is cast on a victim, he regains one point of strength and constitution per day, up to his normal score. The censer acts as a flail in combat.
Plague monks are dangerous opponents. For one thing, they have 2 Hit Dice. They also may cast the following spells once per day: Curse, Cause Light Wounds, Cause Fear, Cause Moderate Wounds, and Cause Disease. Plague monks fight with a short sword in each hand. (House Rule: Roll 2d6, take the highest number.) The Cause Fear spell forces the target to make a Saving Throw or run away from the Plague Monk as fast as he can for 1d6 rounds.
Packmasters are skaven who have an innate ability to command rats, equivalent to a Charm Animal spell. They will be leading 4 - 16 giant rats when encountered. If in an area populated by rats, they can summon an additional giant rat each round along with engaging in normal combat. Packmasters are the only beings able to cause rats to attack skaven; even human mages or clerics can't do this. Packmasters have 2 HD, with maximum hit points. They fight with a short sword in one hand and a whip in the other. (House Rule: Roll 2d6, take the highest number.) Packmasters also command Rat Ogres, once they are experienced or have enough prestige within their clan.
Jezzail are skaven who have mastered the art of the rifle. They are the only creatures that know the secret of creating smokepowder. They will never reveal the secret of its creation and appear to have some type of mental block when questioned concerning smokepowder. Spellcasters who attempt to discover the secret will meet with no success. Jezzail are totally dedicated to their weapons, and will wield only their rifle in combat. They disdain all other weapons. A unit of jezzail firing in unison can decimate the best knights, as some humans have learned too late. Jezzail will go to any length to recover lost weapons. If a human is seen with a gun, he will be attacked at the earliest opportunity (like when he goes to sleep).
Jezzail Rifle: Dmg 1d6, ROF 1 per 2 rounds, Range 70 ft, Weight 8 lb, Cost -, Armor only provides half protection (ex. plate mail only provides -3/+3 armor bonus).
Grey Seers are extremely rare. There are only about 20 of them in any given world. They have 5 HD, but cast spells as a 10th level mage. They have a bodyguard of 30 stormvermin with them at all times. They are the masters of the skaven race, and all skaven defer to them. Seers maintain a library and a laboratory much as human mages do. Each Grey Seer has his own selection of spells, but they all tend to prefer spells that cause lots of damage (such as fireballs), as well as using curses and spells that cause disease or prolonged pain.
Rat Ogres are huge, mutated rat men. They spend much of their lives in a berserk rage, as they live in a constant state of pain. They are also not very intelligent. A packmaster will accompany any Rat Ogres encountered, keeping them under control and preventing them from slaying the skaven nearby. Rat Ogres are statistically identical to normal Ogres.
Stormvermin are the skaven warrior elite. They average 6 feet in height and are quite muscular (+1 to hit and damage with a weapon). They have 3 HD, with maximum hit points. They wear plate mail (AC 3 ) and wield two-handed swords, great axes, and halberds in combat. They are immune to fear (including spells, dragons, etc). They are often used as bodyguards by the clan elite, as well as being excellent shock troops in battle. Stormvermin typically aren't too bright, and as such are rarely encountered in any type of leadership capacity (not counting leading other Stormvermin).
Warpfire throwers work in groups of two, carrying their famed weapon. A warpfire thrower spits out a ball of green fire 50 yards, which explodes and injures everything within a 10-foot radius. Those caught within the blast radius take 3d6 points of damage or 1d6 damage if they make a saving throw. Warpfire throwers are the only beings who know the secret of their weapon, and appear to have a mind block similar to the Jezzail. Note that a thrower cannot be fired during any round in which its bearers have moved, and can only be fired once every 3 rounds in any case. If one of the bearers is killed, the remaining skaven can still fire the weapon, but will be unable to move it anywhere. Warpfire throwers will let no other skaven touch their weapon and will destroy it if it cannot be removed from danger. Humans who manage to capture an intact warpfire thrower will be unable to get it to work. In fact, in all such cases where this is attempted, the weapon explodes, causing 20d6 damage to all within 40 feet. While the warpfire thrower is a powerful weapon, any medium level mage can easily best it with fireballs or lightning bolts. As such, warpfire thrower teams either avoid mages totally during combat, or attempt to sneak up on them and kill them with one shot. A warpfire thrower looks like a World War II flamethrower, with one skaven carrying the fuel canisters on its back and the other holding the tubing and nozzle. These skaven also arm themselves with short swords in case they are forced into melee.
Skaven are extremely territorial, and often war with each other. A different clan rules each area, and if members of another clan enter the area, it is considered an act of war. These wars are often very vicious, and are one of the main reasons the skaven do not overrun everyone just by numbers.
Skaven also fight amongst themselves to determine rank within the clan. These fights are seldom fatal, but all skaven are scarred, with many missing an eye or ear. The Grey Seers encourage this behavior, as the Horned Rat says that only the strong may rule. Grey Seers are above inter-clan warfare, and often visit each other. No skaven would dare assault a Seer or his bodyguards. Seers also work together, though not well. There is much politicking among them, but they would never even consider killing another Seer. The skaven are also slavers. They enslave those they defeat in battle, forcing them to do all the menial labor.
I also have a pdf of this located HERE