The Unearthed Arcana defines a new list of feats called Weapon Group proficiency feats. Rather than having all weapons split into three categories: Simple, Martial, and Exotic, weapons belong to one or more smaller groups, and characters gain proficiency in smaller groups of similar weapons.

Using this variant rule, any of the weapon-specific feats (Improved Critical, Weapon Focus, and Weapon Specialization) apply instead to a weapon group. The normal prerequisites for such feats still apply.

Weapon Groups

Type	Normal Weapons	Exotic Weapons	Exotic Double Weapons
Axes	handaxe, battleaxe, greataxe, dwarven waraxe	dwarven waraxe (one-handed use)	orc double axe, dwarven urgrosh¹
Basic Weapons	club, dagger, quarterstaff
Bows	shortbow, longbow, composite shortbow, composite longbow	elven double bowA, greatbowW, composite greatbowW
Claw Weapons	punching dagger, spiked gauntlet	bladed gauntletA, claw bracerA, panther clawA, stump knifeA, tiger clawsA, ward cestusA
Crossbows	heavy crossbow, light crossbow, repeating heavy crossbow, repeating light crossbow	great crossbowA, hand crossbow
Druid Weapons	club, dart, quarterstaff, scimitar, sickle, shortspear, sling, spear	greatspear
Flails and Chains	light flail, heavy flail	chain-and-daggerA, scourge, spiked chain, three-section staffA, whip, whip-daggerA	dire flail, gyrespikeA2
Heavy Blades	longsword, greatsword, falchion, scimitar, bastard sword (two-handed use)	bastard sword (one-handed use), mercurial longswordA, mercurial greatswordA	double scimitarA, gyrespikeA3, two-bladed sword
Light Blades	dagger, punching dagger, rapier, short sword	kukri, saparaA, triple daggerA, war fanA
Maces and Clubs	club, light mace, heavy mace, greatclub, morningstar, quarterstaff, sap, warmaceW (two-handed use)	warmaceW (one-handed use), tonfaA	double maceA
Monk WeaponsI	kama, nunchaku, quarterstaff, sai, shuriken, siangham	butterfly swordA, tonfaA
Picks and Hammers	light pick, heavy pick, light hammer, warhammer, scythe, maul (two-handed use), lucerne hammerA	dire pickW, gnome battlepickA, maulA (one-handed use)	double hammerW, gnome hooked hammer
Polearms	glaive, guisarme, halberd, ranseur	heavy poleaxeW
Slings and Thrown Weapons	dart, sling	bolas, chakramA, gnome calculusA, halfling skiprockA, net, orc shotputA, shuriken, throwing ironA
Spears and Lances	javelin, lance, longspear, shortspear, trident	duomA, greatspearW, harpoonA, mantiA, spinning javelinA	dwarven urgrosh4

^IMust also have the Improved Unarmed Strike feat.
¹Must also have proficiency in Weapon Group [spears and lances]
²Must also have proficiency in Weapon Group [heavy blades]
³Must also have proficiency in Weapon Group [flails and chains]
⁴Must also have proficiency in Weapon Group [axes]
^AWeapon from Arms and Equipment
^WWeapon from The Complete Warrior

By taking a Weapon Group feat, a character may use any of the Normal weapons listed in that group without penalty. A character using a weapon from a Weapon Group in which he has no proficiency takes a -4 penalty on attack rules.

In order to use the Exotic or Exotic Double weapons without a -4 penalty on attack rolls, additional feats are required.

Special Weapon Group Feats

Feat	Prerequisites	Benefit
Weapon Group (Exotic Double Weapons)	Base attack bonus +1	You may use the exotic double weapons from any Weapon Groups you have mastered
Weapon Group (Exotic Weapons)	Base attack bonus +1	You may use the exotic weapons from any Weapon Groups you have mastered

Taking one of these feats allows a character to use the whole list of Exotic or Exotic Double weapons from all of the weapon groups she has mastered.

Characters gain weapon group proficiencies whenever they take the first level in a new class. This applies even if the class is not the first class a character has taken: just as a rogue character taking a level of Fighter learns to use heavier armor and shields, and gains feats to account for it, a character training in a different class can be expected to learn to use additional weapon groups.

Class	Weapon Group Proficiencies at 1st level
Barbarian	Basic weapons, plus any other three
Bard	Basic weapons, plus any other two
Cleric	Basic weapons, plus any other two
Druid	Basic weapons, plus either druid weapons or spears
Fighter	Basic weapons, plus any other four
Monk	Basic weapons, plus any other one
Paladin	Basic weapons, plus any other three
Ranger	Basic weapons, plus any other three
Rogue	Basic weapons, plus any other two
Sorcerer	Basic weapons, plus either spears or crossbows
Wizard	Basic Weapons or crossbows

Weapon Group (any) is available on the Fighter's list of bonus feats.

Finally, certain races gain some free weapon proficiencies or are naturally familiar with specific exotic weapons.

Race	Prerequisite	Extra Weapon Group Proficiencies
Drow		Weapon Group(crossbows), Weapon Group(light blades), hand crossbow
Dwarf	Weapon Group(axes)	dwarven waraxe, dwarven urgrosh
Elf		Weapon Group(bows) and either Weapon Group(heavy blades) or Weapon Group(light blades)
Gnome	Weapon Group(picks and hammers)	gnome hooked hammer

Final notes:

1. The Weapon Groups listed in Unearthed Arcana omitted several weapons from the Player's Handbook. I have assigned them above into appropriate groups: Morningstar (Maces and Clubs), Two-bladed sword (Exotic Double Heavy Blades), and Net (Exotic Thrown Weapons).
2. Drow did not have a distinct list of racial weapons in Unearthed Arcana but the Forgotten Realms Campaign Setting said that drow are proficient in short sword or rapier, as well as light and hand crossbow, rather than the usual Elven weapon proficiencies. The list I gave seems compatible with that.