Be sure to click on the link to the album and then go down to the samples near the bottom of the page. "Angel" is the first cut listed.
As usual, we wouldn't have been able to track down this song this quickly without information posted on the Television Without Pity West Wing Forum
Later, we heard from Warren Bakay who emailed us the following: "Tonights episode's (Commencement) song at the end is 'Angel' by Massive Attack, off of the Mezzanine album (1998) I can't get over how well chosen the songs on this show are. Absolute Brilliant."