I love listening to music while I work, and free music is awesome. NPR has a series of full live concerts that you can listen to online or download for free from their series All Songs Considered podcast
These are great, because you can just push play and have an hour or two of great music to work to. You can also download the MP3's from their podcast (link).
My favorites include:
They also have the complete lineup of festivals like SXSW and the Newport Folk Festival for download, so you can re-live the experience there. I find googling an artist with NPR to see what they have for that artist. For example, if i wanted to find a couple Bon Iver concerts they have, I'd just google "NPR Bon Iver"
Happy Listening! let me know which ones you enjoy