Google says there are:
- Jenkins ( first impression: not lightweight )
- Cruise control ( first impression: not lightweight )
- Bitten ( seems to be dead, last ticket >8 months old)
- BuildBot
But BuildBot seems a bit outdated, offering no statistics, not very intuitive to configure... Features we are interested in are:
- Multi platform support for build slaves: Linux, Mac OS X, Windows
- Easy build slave setup
- Mercurial support with branch sensitivity
- Scheduled builds
- Commit-hook builds
- Email notification with full debug output
- Trac integration would be nice but is not a must-have feature
Edit: I favour Jenkins over Hudson by now, since its the real open source fork of Hudson. Oracle just failed.