What do you guys use?

Besides ModernBill, PerlBill etc..

There's PHPCoin http://www.phpcoin.com/ which looks promising. Definitely not as feature rich as ModernBill and some of the others, but like they say: you get what you pay for.

Looks interesting, ugly, but interesting

You can make your own skins for it if you're artistic and design oriented, heh. If you visit the forums for it some people have done some neat things with it.