Prior to defecting to Sibelius, I was a dedicated 'mup' user:http://www.arkkra.com/
Fully functional binaries for Windows and Linux may be downloaded for free from Arkkra's web site. Registration is $29 (unregistered copies add an annoying watermark to printed output.) Source code is also available, and compiles easily on most platforms. Contributed binaries are also available for other platforms, most notably MacOS 9.x and 10.x.
This is a text-based command line utility similar to Lilypond that generates PostScript output. When I evaluated the two side by side I chose 'mup' over Lilypond because I found its notation language friendlier and more intuitive. In my experience (on MacOS 9.x and Linux) the code is rock solid.
For raw entry of a score, I much preferred the text-based environment to the more common WYSIWYG environment of the popular commercial packages. Beyond that, however, I find that most tasks are more easily accomplished in a WYSIWYG environment.