Ranked in these QuestionsQuestion Ranking
Pro Committed to free/open licensing
Pro 2D and 3D art
OpenGameArt has both 2D and 3D art assets.
Con Assets may have restrictive licenses
OpenGameArt hosts art under any of several licenses, some of which may not be suitable for commercial projects or for release on certain distribution platforms. Careful review of which licenses are compatible with your project is essential. The site's search facility allows filtering by license.
Con Mediocre organization and search functionality
The curator of OpenGameArt acknowledges that the tools for finding the assets you need are sub-par. Correcting this is a near-future priority for improvement, and as of September 2014 is the next milestone goal of the site's Patreon campaign.