One of the easiest ways to enhance a Monk's unarmed striking power isn't through magic items, but other pathways.
Multiclassing to certain PrCls like the Kensai (CompWar), for instance, boosts the PC's weapon of choice- which can be a Monk's unarmed strike- and has other benefits as well. And it is a PrCl that doesn't cut off Monk advancement. Similarly, the Shiba Protector Monk PrCl (Oriental Adventures) lets the PC add his Wis bonus to attacks and damage.
The Improved Natural Attack feat also boosts the Monk's potency.
If your campaign cherry picks stuff from 3PPs, you might want to check out Monte Cook's Arcana Unearthed/Arcana Evolved. In it, there is a feat called Hands as Weapons, which lets the Monk enhance his unarmed strikes in exactly the same way as manufactured weapons. Some people allow this in standard D&D on the basis of this language:
Not everyone agrees with that interpretation, of course...which is one reason why Monte Cook put that language in his game.