Great fan remake of Goldeneye from the N64, been about 5 years in the making and is due for release today - for free! It may appear on steam later, but currently available for preload and the OST they created for their new version is also available from moddb!
You don't need Half Life 2, you need Source SDK Base 2007 installed. It's free on Steam, to get it you go to Library > Tools, scroll down to Source SDK Base 2007 and install it. Then install the GoldenEye Setup .exe file, and you're good to go. The game will appear in your normal Steam library just like your other Steam gameshttp://wiki.geshl2.com/goldeneye/installation
-ssimonian (quoting anastazius below as well)