Asda are selling Half Life 2: The Orange Box for the PC for just £15.93 (inc Del.)
You can also get cashback through the usual sites (e.g. 2.5% Quidco, making it £15.53)
The next best price is £23.49 from SoftUK.
NOTE: At the time of posting, I forgot that Asda's website is operated by TheHut, so you can now get this game for the same price from them. If you use Quidco, TheHUt gives 5% cashback, which makes it a slightly better deal (£15.13).
Half Life 2, Half Life 2: Episode 1, Half Life 2: Episode 2
The newest installment in the best-selling and highest-rated action game series of all time. Battle creatures sprawling above and below ground in the White Forest. Then, take a thrilling ride with Alyx in a new modified vehicle as you enjoy the most breathtaking and revealing Half-Life game ever made.
Open your mind to a new way of gaming. Portal blends puzzles, first person action, and adventure gaming to produce an experience unlike any other. With the Portal device, manipulate your surroundings to complete dozens of unique challenges. Are you ready to start thinking with Portals?
Team Fortress 2:
The sequel to the granddaddy of class-based multiplayer gaming. Join and battle teams with wildly-varying player classes - from the flame-throwing Pyro to the room-clearing heavy weapons guy. Featuring a distinctive visual style, Team Fortress 2 features all-new multiplayer game modes.