Borderlands PC download, only £2 @ Green Man Gaming.
Use the following code to bring the price down by 20% to £2:
This is the cheapest I've ever seen it, including on steam. The GOTY version is also a fantastic price at £3.99. I assume they both require steam (only the GOTY Page mentions it)
If for some reason you don't have it, for the love of god, BUY!