I used the Tektro Onyx. very cheap and fairly effective. Maybe a bit heavy, and maybe not the most braking power.
Other cheap ones are the Avid Shorty 4, 6 or 8(ti). Someone said they had a problem squealing.
I was thinking about these Empella Frogglegs as an upgrade. Other options are the Paul Neoretro, Spooky and Shimano make some I know little about.