Probably the most fun you can have with any Half Life 2 mod. For the cheap price of 10$ this game is so worth it... What is it? It is many, many things, for multi-player there are different modes, such as zombie survival, build, winter survival, and others.

Single player, my personal favorite, is a big sandbox where you can do anything, (when I say anything I mean anything) you want to. Addons are extremely awesome because there are so many.
Download site for addons:
http://garrysmod.org/downloads/

Tutorial how to import addons:
http://ultimategamers.darkbb.com/tutorials-f44/the-real-way-to-get-addons-t295.htm
Pros:

• Unlimitedness on what you can do.
  
• garrysmod.org can provide anything you need.
  
• Multiplayer is fun too.
  
• Only 10$
  

Cons:

• Sometimes has major lag.
  
• Requires HL2.

My rating : 9.8/10