Logged onto my Steam account today and found that Garry's mod Had a 50% Sale for One Week Only! Reduced from £5.99 to £2.99! What a Deal! Highly Recommended. I Have it and have some great fun on it!
Warning: It does require a source game to run, Such as half life 2 (and Onwards) , portal, counter strike source or Team Fortress 2. So you would have to buy another one of these games. but you can buy it in a package as listed on the page! So you would be saving money!