An article from the Houston Chronicle reports that although the liberal group MoveOn had their Superbowl ad rejected by CBS, they may have benefited more from the publicity about the ad rejection than the ad itself.
Among other things, they saved the $2M placement fee and have now garnered almost 40,000 new members since the word started getting around the Internet that the ad was banned.
CBS claims that they had a problem with the ad because it was clearly and advocacy piece. By the same token, they have rejected another ad by PETA (People for the Ethical Treatment of Animals), a group known for their vehemently anti-fur stance (among other things).
Moveon.org is a liberal organization that has been pushing against the Bush administration since it got into office. Originally founded during the Clinton administration to push the idea to "Censure President Clinton and Move On to Pressing Issues Facing the Nation." (From their about page). The group doesn't claim any political affiliation, but I can't find any instance of backing non-Democratic candidates or issues. Not that there's anything wrong with that, just seems like they should have some kind of disclaimer somewhere.
That notwithstanding, the group sponsored a contest to develop the ad to be aired during the Superbowl and got quite a few submissions. Bush in 30 Seconds contains the top four on its main page and access to the 26 finalists. I've got to say that the quality of the top four submissions is quite good. Even if you don't appreciate the sentiment (and, frankly, many of the points they make are on-target), the care taken to construct these is impressive.