Today’s Rolling in the Higgs thread on MetaFilter sent me on a tour through Youtube and Vimeo…
Tim Blais aka A Capella Science (Tim explains about it here): Rolling in the Higgs (Adele Parody). Tim also is one half of Cabin Nine, and they cover songs like these: Europe: The Final Countdown (with guitar and cajon!), Amy Winehouse: Rehab.
AstroCappella is a project by The Chromatics: NASA | The Sun Song.
And of course this oldie but goldie: CERN Rap (also on Youtube, lyrics here).