Garry's Mod is a tiny (3 mb) mod that lets you mess with the physics in HL2. You can do pretty much whatever you want to do...it is very addictive.
You can view more about the mod here:
http://www.garry.tv/garrysmod/
And a couple videos I have creating using the mod here:
http://ignt.filecloud.com/files/file.php?user_file_id=8810
http://rapidshare.de/files/861333/igjet_hi.avi.html
^ I'm not sure if the rapidshare.de link works or not...it doesn't for me. You only upload to it through your browser, and I'm not sure whether it timed out or not.
Anyways...enjoy the mod/videos

