What are the free programs available to typeset music?
Handles big scores with a lot of instruments, has many layout options and some bugs. Much of the user interface resembles Sibelius. You can export/import in numerous formats, too (MusicXML, MIDI, ...). Runs on Linux, Windows, Mac. It is being updated and developed regularly.
GPL Notation program built on lilypond.
Online and there's a free level that's quite good. I've been using this to swap ideas with writing partners across the continent for about a year now and it's great. The interface has undergone a few revisions now and every time come out better and better. I can move pretty quickly with it now and it's nice to be able to access and work with my lead sheets from any place I can get a browser connection.
More like a sequencer. One shouldn't expect too fancy layout options from it.
Seems to be rather limited in score layout as well.
ABC Notation: not really a software in itself (even if some software encode and decode ABC Notation) -- comes with a free software toolset, and can be for example exported to Latex.