An addon that lets you use mathematics inside of Gmod. Calculate stuff. Make automatic stuff. Automatic turrets, doors, elevators, robots, whatever. There is also the almightly Expression 2 that literally lets you code inside Gmod. With Expression 2 you can make anything.
Life Support & Resource Distribution is used in the Spacebuild gamemode and adds a twist to your game. In spacebuild, you are on space maps with planets. If you go off the spawn planet, you will float freely in the gravity less void until you suffocate. To survive in space and on other planets, you need to build some life support.
SVN is a program that lets you download things and extremely easily keep them up to date.
If you use Gmod.org or glua.net, you must re-download the entire addon every time there is an update. That can take up to 30 minutes if your internet is not very fast. With SVN, you just have to download the 1 or 2 files that have been changed to fix that one small bug. That will take just a few seconds.
Here is an SVN tutorial: http://www.facepunch.com/showthread.php?t=688324