• LilyPond is an automated engraving system. It formats music beautifully and automatically, and has a friendly syntax for its input files. It is Free Software (‘open source’). More information you can find here and at the Lilypond page of wikipedia .
• An nice article about comparing Finale with Lilypond.

Rosegarden,NtEd, Canorus.

* NtEd is a new musical score editor for Linux, with a Lilypond export function. It's totally WYISWYG and very fast. It's pc-keyboard insert function is similar to the 'quick insert function' of Finale. NtEd does have a unique midi import function and much more.

• Musescore or Mscore is a free cross platform WYSIWYG music notation program, licenced under GNU GPL. This program works on Windows too and is comparable with Finale and Sibelius. Here you can find more information.
• First impressions about Mscore. First impressions about Mscore.

• Rosegarden is a well-rounded audio and MIDI sequencer, score editor, and general-purpose music composition and editing environment. You can use Rosegarden to make Lilypond files. Rosegarden is an easy-to-learn, attractive application that runs on Linux, ideal for composers, musicians, music students, and small studio or home recording environments. The score editor of Rosegarden is great and much improved in the latest versions.
• Here you can find an review of Rosegarden 1.5 by Dave Philips.