TTT is pretty friendly to modding/extending in various ways. You can create new weapons, new maps, modify weapon spawns on existing maps, etc. Here you can find some guides and information on modding and mapping for TTT.
How to create custom weapons for TTT.
Documentation for TTT-specific hooks available if you are making addons in Lua.
How to get started making maps for TTT.
(In)frequently asked questions and problems encountered making TTT stuff.
How to use the weapon placer to create rearm scripts that change weapon spawns on maps.
How to translate TTT into a new language.