Trouble in Terrorist Town (TTT for short) is a really awesome gamemode, and is very unique.
In case you want a quick overview, a small group of people called Traitors try to kill a larger group of Innocents, but at the same time no one but the Traitors know who the Traitors are.
The Traitors get special tools and gadgets to help them kill the Innocents but since there are more Innocents the goal is for the Traitors to pick them off one by one in secret to take them all out, but there are usually one or two special Innocents called Detectives who have access to their own gadgets and their job is to track down the Traitors using DNA evidence found on dead bodies and other tools.
Official website: http://ttt.badking.net/
Requirements: Counter Strike: Source
You don’t need to download it because it comes with gmod by default now, though you’ll have to join a server to get the maps.