A (totally unneeded) reason to replay Half-Life 2!
http://kotaku.com/5441126/modders-drag-half+life-2s-graphics-kicking--screaming-into-2010

And another!

http://www.moddb.com/mods/research-and-development

"A puzzle-centric mod that features an unarmed player but plenty of violent mayhem."

So far I'm enjoying it immensely.

I dunno if any of you out there have messed with this, but it's awesome. Especially the real working stargates you can get. And the spacebuild submod where you try to contruct a real, life-sustaining spaceship and then attempt to get it off the ground.