mt-daapd is a program which acts as an iTunes (DAAP) server for Linux
and other POSIX unixes. It may eventually support Win32 as well.

Some random features that it supports:
Supports the Roku SoundBridge (query/browse/index support)
Supports periodic rescans of the database
Can advertise shoutcast streams
Supports password-protected shares
Is web-configurable and managable
Support dynamic playlists, like iTunes "Smart playlists"
Probably other things I've forgotten

All is not perfect, however. Currently, there are several drawbacks to it:
database is indexed by inode, so all music must be on one filesystem
browse by artist is slow on Roku, times out (with large databases ~10K songs)
Probably other things I've forgotten
