Are you among the lucky first thousand?
With Steam's Family Sharing feature, you can lend your Steam library with "family members" who are on authorized devices and all parties involved retain their own achievements and save files. A nice addition, certainly, though it's not quite here yet -- at least not for most of us.
As promised, the beta has rolled out. The first thousand invites have been sent out via email. If you'd like in on this action, join the Family Sharing Group on Steam. Perhaps more importantly, there's now a user guide with helpful information for the rest of us plebeians while we're stuck here waiting. It's something of a disappointment that entire libraries are shared as opposed to individual games, but that's to be expected. Truly, that would be outrageous.