I have iTunes at work, but there's nothing on it. Bringing my iPod here and copying songs to the computer is a pain and a half in the ass, and I have neither the inclination nor the disposable income to go on an iTunes Music Store spree just to have stuff to listen to at work.
But holy shit, Batman, there's a lot of free music out there! And not the illegal kind that'll get you sent to federal pound-me-in-the-ass prison, either!
First of all, Salon.com. Doesn't exactly count as free, I guess, but if you have a membership they give you a bunch of free MP3s to download every month or so. And for non-members, you can do the "free day pass" option and click over to Wednesday Morning Download, which also has a bunch of free stuff.
Then there's CNet Music, which (despite looking somewhat scary and corporate) has a virtually endless well of music sorted by genre. Even some pretty well-known stuff; you can search an artist and see what they have.
BetterPropaganda.com I haven't looked through extensively, but it looks cool and appears to also have a reasonable amount of free shit.
I'll add to this list if I find more good places.