Dystopia released their Half-Life 2 mod for download a couple of weeks ago and the response has been incredible! From Fuzzy:
Our release weekend was huge, we've had over 100,000 downloads of the demo and we cracked a peak player count of 2000; higher than HL2DM! We're currently vying for the number 6 most played Valve engined game on the net. Easily the most played mod.

Dystopia is a perfect example of hobbyist developers coming together to build a really fun project with Visual Studio. If you want to build your own HL2 mod you can get started at our Coding4Fun Half-Life 2 Mod Spot.