Agreeing with Niels: mood is important.
My musical taste ranges from mainly 80's to modern days, from the more 'commercial' hip-hop (never the hardcore stuff though! Must sound cheerfull and happy) via alternative to metal at times.
Jonathan, tip perhaps if you like good guitar music: check out Steve Vai. Lately re-discovered a song called 'The audience is listening' from his (already bit older) album 'Passion and Warfare'... Hilarious!!

(found it on ITunes again)
And then ofcours there is still Marillion (mainly their bit older work) which can make me dream away...
Good post: makes me want to check out my cd collection again...

Aloha,
Stef