The newest release of the Trouble in Terrorist Town gamemode is included in Garry's Mod. This page lists the detailed changes in every release. If for whatever reason a version is released in between GMod updates, for example to fix an urgent bug, the post will include a download.
Looking to play the game?
You do not need to manually download and install anything if you just want to play TTT. Just join a Garry's Mod server that lists "Trouble in Terrorist Town" as its game in the server browser.
Want to host a server?
Install garrysmod and CS:S on your srcds. GMod includes a recent version of TTT. Set the "gamemode" cvar to "terrortown" and load a compatible map such as any standard CS:S map, or one of the maps made specifically for TTT that can be found here.
Looking for an SVN?
You have a few options: