Oriental Adventures is really great for expanding Monks. Even without prestige classes, Monks from OA get a slightly expanded weapon list, the Knowledge: Religion skill, freedom to multiclass, new martial arts feats, access to martial arts styles, and the option to swap out certain class features for bonus feats.
There are 3 Monk prestige classes in OA, including 2 of my favorites. The Shintao Monk gets bonus martial arts feats and some nice abilities. The Tattooed Monk gets magic tattoos every other level, which allow some nice abilities like ability boosts, damage reduction, and so on.
I don't have Quintessential Monk yet, so I can't comment on that one. I've heard really good things about it, and plan to get it soon.
Path of the Sword has some interesting Monk prestige classes. There's the Ki Warrior, that focuses on using ki in special ways, but I like the concept more than the mechanics. From a mechanical perspective, they just seem like an odd mish-mash of speical abilties that don't necessarily have anything to do with ki at all. Swinging from vines? Poison fist? But I really like the Shadow Tiger from that book. It's a monk that focuses on stealth and a tiger-like fighting style.
There was also a really cool Monk PrC by Monte Cook from Dragon #296. It's called the Acolyte of the Fist, and is focused on striking power and mobility.