I guess I will be the first to mention Luntbuild. Aside from the less-than-catchy name, it's a great CI server.
- Super easy to setup.
- Web UI for creating/managing your projects and users
- Support for LOTS of version control systems (I've used it with CVS, SVN, and StarTeam)
- Pretty slick remote API
- Relatively high granularity for access control (so you can give clients access to only their builds, and not toher clients' builds)
- and much much more.
Check out the Live demo.
account/password demo/demo
Note: The live demo is a few releases back.
