Robin Good: UICloud is a curated collection of some of the best user interface elements, available under a Creative Commons license. The collection which includes already over 20,000 designs provides an easy-to-use search engine as well as a set of browsable categories to easily find specific UI designs.
Another great example of how useful it can be to have someone that picks, collects and organizes sets of valuable items otherwise scattered all over the web.
N.B.: Anyone can submit design elements to UICloud, but each new submission is reviewed, evaluated and, if accepted, appropriately categorized, by human reviewers. See: http://ui-cloud.com/submit/
Curation at work.
Try it out now: http://ui-cloud.com/