In these days of photo-realistic 3D games, it's hard to remember programs such as Castle Wolfenstein that started off the whole genre 20 years ago. If you want to relive your childhood, or you want to see what those original games were like, or you just want to play a simple, fast, fun 3D shooting-based adventure game, point your browser at http://wolfenstein.bethsoft.com/.
It's a full port of the original game. The graphics are still crude, but the sound is great and it runs in your web browser with no need for a download. It's free, and supports major browsers including Chrome and IE.
Thanks to George for telling me about this neat find.