I have seen a lot of HL2 modifications. Some have extensive development teams, and put a LOT of work into their projects. Then others kinda' suck, and don't put 100% effort into their work.
BUT! I've kept my eyes open and I am really looking forward to Black Mesa:
For all of you noobs who didn't understand the damn video, Black Mesa is a HL2 mod that places the original Half Life game into the Source engine (used to design Half-Life 2).
Release date is set for 2009, but I have a strong feeling that Valve is going to buy Black Mesa and sell it.
Please, God, oh please :)
More updates soon! be sure to check out the website for first-hand info!
http://www.blackmesasource.com/