I just got garry's mod and i love it! i think it must be one of the most fun and useful games i have ever played. You have a wide array of tools to use, over 300 diffirent props, and you can download new addons and other stuff all the time! If you've never heard of garry's mod it's a game that runs by using any games made with the source engine, ie: Hl2, Counter strike source, or any other half life game. You can build almost anything with gmod and i really do suggest you get it! it costs $10 (5 pounds). If you have Steam then you can buy it off that if not buy it from the actual site.

www.garrysmod.com
www.garrysmod.org
go to .com to buy etc... and .org to download new mods and addons.
Type in garrys mod in youtube to find lots of videos on it.