Minifigures creator?
Minifig Generator might work for you.

Or maybe the SigFig Creator.

Flash based, but also fun: Mini-Mizer.

Even better, suggested by rioforce: LEGO Universe Minifig Wizard

I'm sure there are more...

6 hours later...  OH!  you mean like an actual bit of molded plastic.

For that, probably your best bet is using one of the above to find some approximation and then hit up something like Bricklink to buy the proper parts.