TubePress is an open source PHP library that displays gorgeous YouTube and Vimeo videos and galleries in your blog. It's available as a WordPress plugin or as a stand-alone PHP library, is extremely configurable, and easy to use. Check out a simple demo gallery or explore all of TubePress's features in the showcase.
There are two different TubePress packages available, both of which are open-sourced under the GPLv3.