Stefan Surmabojov walks you through the basics and a render test in An Inside Look At OpenGL Within After Effects at AEtuts.
See also Render with OpenGL, Preview modes and Viewer Quality preferences, and other pages in AE Help on the web, as well as the list of supported graphics cards. There's a few more resources including video tutorials by Mylenium comparing Open GL features in AE 7 & 6.5, a demo for AE 7 by Steve Holmes and others in the major feature reviews at Total Training and Lynda.com. Unique factoids, like only the first 8 lights will render with OGL in AE, might only be found in Creating Motion Graphics by Trish and Chris Meyer.
Maybe Toolfarm will come up with a list of 3rd party plug-ins that support OpenGL or hardware acceleration, like the recent releases by GenArts. Anyway, here's Stefan's video (downloadable at AEtuts):