Just took a look on PlanetHalfLife and found that Rock 24 has been released. You're Gordon, again, and this time you've got to rescue a rebel scientist named Richard G. Newell. And while you're doing this you've got to save yourself from the confines of a mountainside prison facility. It says that the playtime is about an hour and a half. Here's the website http://developer.valvesoftware.com/wiki/Rock_24
Hmm...looks interesting.I should try it someday.
It was worth the download. Hope it's worth the play now. ;)