I was just using a program to look around and find out what's taking up the most space on my tiny little hard drive.
In the Steam/Dumps file; I found 44+ GB of essentially useless files for games I had thought I uninstalled. Given my second hard drive is broken I only have 100GB and was wondering where all my space went.
L4D2 alone took up 14GB of space but I don't think that's just the Dump file; and apparently those are just all developer stuff that you can refer to when the game crashes or whatever. Again all these games have been uninstalled except for Portal2 and ZPS, just not all the way I suppose. Didn't know they stuck around like that.
If nobody knew of this or doesn't already have a problem that deletes that stuff; take a look and see if you can free up some space.
Of course for games you have installed it'd be wise not to delete it all; but I haven't played L4D2 in months and yet there was 14GB of crap on it. My ZPS section in the /steam/dumps section though holds all the demo files and whatnot; so if that's how it works for Gmod I'm sure the admins know about this somewhat. Maybe they never noticed how much space can be taken up though.
