For some reason, Half Life 2 inspired fashion hasn't quite kicked off (the Steam Store sells Headcrab hats that are quite dapper, but there are no HEV suits in my local fashion outlets, but some make their own ) while constant checks of the gadget websites continue to frustrate attempts to buy a gravity gun.
With literary works also available, the only real thing missing in Half Life 2 fandom is a real movement of fan-made music inspired by the game. One guy who has started the ball rolling in this field is Jonathan Lewis whose song G-Man's Last Breath plays like an excerpt from a rock opera, and features the lyrics:
Did you ever wonder why
G-Man’s what they call me
Just take the “G” from you first name
And from you last drop the “Free”
Gordon drop the “Free”
Hopefully there will be more to come along those lines - after all, Harry Potter has Wizard Rock (WRock), so why not HLRock?