Gmod is great.
I guess you mean "now" huh? I was going to say there's one that has been in development for a LONG time called Black Mesa, which is a reworked HL1 Source using better models and everything from HL2.
Best $50 I've spent was on Orange Box. So many games for so little.
Don't you dare bash Steam either. Valve has improved it so much over the past few years. When it first came out, everyone thought it would just bend over and die and look at it now. It's stable, clean, fast and easy to use. It's the iTunes of games but without the code bloat.