If you’re obsessing over which Steam Trading Cards you don’t have for whatever sets you’re collecting, and you know that your friends have some of the ones you need, then you’re in luck: Trade Offers are now available, and they’re the digital equivalent of “I’ll give you three of these for that one.”
Trade Offers let you send a trade offer (surprise) to a friend – whether they’re online or offline – and they can respond to it in their own time. You can pick what you want from their collection, and choose what you want to offer in return; they can then accept, refuse, or make a counter-offer.
You can’t do this if your friend has their inventory set to Private, obviously, because being able to see what cards they have while not being able to see what cards they have would probably make the universe implode.
Trade Offers are good for two weeks, and you can currently only send them to friends, although Valve reckon they “may expand this in future.” It’s also worth noting that you can actually make several offers using the same cards, and as soon as the first one is accepted, all the other offers will automatically vanish. As an example, if there’s one card you really need and three of your friends have it, you can offer all three the same deal, but only the first deal accepted will be honoured.
This all assumes you’re actually interested in the Steam Trading Cards, of course. It’d be nice to see used with things like Dota 2 and Team Fortress 2 items, but definitely only for friends – I already get bombarded with requests for some of my rare Team Fortress 2 items.