Apple is a strong supporter of open source (and much of apple's software is based on open source stuff), so most apps have either been ported already (and available for download from apple) or can be recompiled for OSX using the GNU compilers with minimal effort. Check out:
http://www.apple.com/downloads/macosx/unix_open_source/
I see that one of my fave app LyX is a "featured" headline item.
