Over on the Steam news page the free Borderlands 2 weekend was announced, so head on over there to grab the game.
If you find you like the game and want to play more of it, you can buy it in the 2K Games sale too for 75% off, making it just £4.99 ($4.99). The Game of the Year edition is also on offer, and that includes all the DLC, so it's probably best to grab that for a slightly higher price (£8.74/$9.99) if you want to extend your Borderlands experience. Total bargain.
There's also a massive sale on for publisher 2K Games' titles, where you can pick up the first Borderlands game, Civilization V, the entire Bioshock series, and XCOM.
The sale and free time with Borderlands 2 is on until 9pm BST Sunday, so you've got a fair few days to test out the game, and decide which bargains you'll pick up.