Flickr has finally brought back their Creative Commons section. It was down for way too long. You can select a license then search for photos under that license, but you can only browse the 100 most recent photos which is odd. It does allow you to get a general sense of the popularity of each license:

Attribution-NonCommercial-ShareAlike License: 548,480 (53.9%)
Attribution-NonCommercial License: 203,232 (20%)
Attribution License: 116,825 (11.5%)
Attribution-ShareAlike License: 110,426 (10.8%)
Attribution-NoDerivs License: 38,641 (3.8%)

Over 70% choose a license that can’t be used for commercial purposes. They also aren’t eligible to be used on Wikipedia.