Sometimes you need to block out the world in order to work. I find the best solution is to put on a solid pair of closed-cell headphones, like my beloved Beyerdynamic DT770 Pro 250ohm headphones, and spin up some good music. With the right tunes I can dive in and work for hours.
Sometimes, however, your MP3 player has died, or your iTunes library sounds a little stale, and searching for music on YouTube would be too much of a distraction. You want fresh playlists of music, with no ads or interruptions. In that case, I have a few suggestions:
StereoMood (currently under reconstruction) has a bunch of playlists grouped by whatever mood you’re in or want to elicit.
Musicovery uses a neat graph – Calm to Energetic, Dark to Positive – to gauge your emotional level, and produces a music track to suit.
I personally find heavily lyrical music distracts me from code or writing; give me instrumental tracks, classical to electronica, and I can develop for days. What about you? What's your favourite music to code or design to? Do you have a favourite site that feeds your musical need?