There's no such thing as a free Borderlands 2 weekend on Steam, my grandmother used to say. Well, granny, look who's the "stupid-faced, manchild, crushing disappointment to the Walker family" NOW. Gearbox's cel-shaded gabbling co-op ARPGFPSOMGWTF is currently experiencing rather big discounts, and is available to fart around in (after a fashion) from now and all weekend without your wallet shedding a single tear. And indeed save a bunch on all manner of 2K games from now until Monday, including XCOM Enemy Unknown for £3.75 and Enemy Within for £6.60. Or all three BioShocks for £6.80.
If you've been meaning to pick up 2012's Borderlands 2, now's the time to do it. You can get the vanilla game for a fiver, or the GOTY edition, with nine DLC packs for £8.75. Both of those are 75% off. Or you can just play it over the weekend for free, and then run off, laughing, waving your money at 2K and shouting, "I'M GOING TO SPEND IT ALL ON SWEEEEEEEETS!"
Other bargains to be found this weekend are Sid Meier's Civ V Complete, with sixteen extra bits and bobs, for £11.55, and The Bureau for £3.
The "weekend" has apparently already started, which makes my being at work a bit of a disappointment. It does mean, however, that you can get these discounts now, and start playing Borderlands 2 free, now.
Except, something odd is going on at Steam HQ. Rather than promoting this weekend in any noticeable way, it's entirely inaccessible other than from the news post about it. So, if you want to get it working, you'll have to go to this post, and click on "click here to install". It will direct you straight to the installer. Odd.