A big congrats to Clay, Paul, and the other folks at Mashery on their successful launch. It sounds like they offer everything you need to support a public API. (Since I don't have a public API, I haven't tried it.) The key features listed on their web site include:

• API Access Control
• Developer Key Issuance
• Usage Tracking and Metrics
• Developer Community Management
• Interactive Documentation
• Rate Limiting

TechCrunch has some more coverage of the launch:

The future is going to be built out of APIs - though still controversial in some quarters today, in time they will be as common as corporate web sites are now. Who's going to build the series of tubes that makes such a future possible? Mashery is aiming to get into that game early.

Best of luck, guys!