Comment An actual answer to the question! (Score 3, Informative) 299
Long standing member of the Linux audio community here, with almost 20 years experience of recording under all 3 major platforms.
Please end the Mac fanboism and answer this poor guys question!
He's asking about LINUX BASED notation software and synths! I'm sure he's well aware of Macs, REAPER and ProTools etc - not that they do what he's after anyway!
Musescore and Rosegarden have already been mentioned for Linux notation software but there is also http://laborejo.org/ , http://denemo.org/ and http://www.frescobaldi.org/ . Laborejo seems to be the most popular in the Linux world these days. I'm not sure which is the best as I don't do notation very often and I've not tried them all. The last few are basically lilypond GUIs.
As for synths, the best (and most powerful) commercial synths for Linux is Loomer's Aspect. Its unbelievably CPU efficient too. As for open source, there is TAL Noizemaker (my fave), zynaddsubfx/Yoshimi, Amsynth and Triceratops are all worth checking out.
Another good free synth (but not open source yet) for Linux is Tunefish - thats my 3rd fave after Noizemaker and Aspect.
The best Linux Audio distros are KXStudio and AVLinux. As for DAWs (which he wasn't asking about, but just for my 2p) Ardour has lots of fans and many people use REAPER under Linux as its officially supported running under wine but my fave Linux DAW is qtractor. Its the fastest and most lightweight modern DAW. It lacks some whizz bang features of the popular commercial DAWs but you may find it does everything you need it to.