This needs a thread, not just a little mention in the 'my rig is freon cooled and consumes entire coal seams worth of electricity).http://release.blackmesasource.com
Go get it! It's Half-Life 1, for free, redone in the spangly new-ish Source engine. This project has been in development hell since not long after Half-Life 2 originally came out and the bonus of Valve's own Half-Life Source was found to be lacking.
Windows only for the time being but if you can find a mirror that doesn't immediately crap out due to the demand then you're good to go.
Can't find any screenies that meet the forum's stringent standards so go to the website if you want to see how nice it looks.