Technically, Valentine's Day was yesterday in Australia, but that doesn't mean we have to miss out on Valve's "Valentine" sale, which features various co-operative games you can play with friends, family, loved ones and your more intelligent pets.
There's about 40 hours left to go on the sale and while there are some quality titles available, it's one of the smaller sales Steam has run in recent times. All three games from the Borderlands series make an appearance; the first title however is not discounted at all. Borderlands 2 is the cheapest at $US10 while the reasonably new Pre-Sequel is 50 per cent off at $US34.99.
Portal 2 is a good deal at $US5 and some great indies such as Magicka and Dungeon Defenders can be had for around $US3 each. Don't Starve Together is a solid pick-up, though it's a little more expensive at $US17.24.
I was expecting Divinity: Original Sin to be on the list, but alas, it is not. The role-playing genre is instead represented by uh, Gauntlet. Actually, the sale is a bit light on RPGs from what I can tell. Still, you should be able to find something suitable for your particular pairing.
Steam Coop Sale [Steam]