As far as I know (and I'm usually proven wrong by somebody) there aren't any reliable adapters that let you use just 'any old' controller thats plug & play with the PS3 with full compatability, unless it's from a reliable manufacturer.
Without getting into a fanboy/girl flame war...
One company I;ve always liked, as they make smaller controllers for girls and people with smaller hands (which often feature turbo/autofire) are Mad Catz controllers, which usually offer 100% full compatability with *most* titles on the market, but I'd still check them against the latest firmware for compatability on their website.
Sorry for not being more helpful, Mad Catz are the only people I'd trust to buy a non-console-branded controller from to actually use with a console. Unfortunately the consoles aren't as forgiving as PC's with what you plug into them, hence why they get away with such stupid prices for controllers (having a PS3 myself, and still annoyed at the fact the base one isn't rumble-enabled and the one that is, is sold seperately, for moronic money)