Hammer is the tool to mod and create source games. It is basically a the main portion of the source SDK. I saw the title "source and development" and that is what sprang to mind...Call me weird I guess. Thought maybe someone on here was into that O_o
Seriously, Half Life mods? Are we back in 1997? Do you do it? I have the special edition of Unreal Tournament 3, incl. editors and tutorial videos. But I haven't gotten around to try the editor yet.
lol Not half-life in particular but also other source games. Left4Dead Team Fortress 2 Alien Swarm and my particular fav! Garry's Mod. Source modding is dated but super accessible thanks to the amount of documentation. It also has a wealth of resources. Look at me all geeking out.