NOTION Music Notation Software
The instrument sounds included with BIRT! were created using NOTION Music, which uses samples from the London Symphony Orchestra.
Here are a few links related to the BIRT! project and software.
Mozart Music Notation Software
Mozart is another music notation software system, which outputs MIDI files. Members of the BIRT! team have used it on many projects. We swear by it!
Ricci Adamsí Musictheory.net
An online music theory treasure-trove.