thehut.com is selling Half-Life 2: Orange Box for the PC for £17.93 (inc. Del).
You can get 5% off this price using the attached voucher, making it £17.03
You can then get further cashback on top of this through the usual sites (i.e. Quidco gives 5% making it £16.18)
For those that don't know, Half Life 2: Orange Box contains the following games:
Half Life 2
Half-Life 2: Episode One
Half-Life 2: Episode Two
Team Fortress 2
If you already own one or more of these games, Valve allows you to give your extra copy away as a gift to anyone you want via Steam.