Valve has announced Steam Family Sharing which will allow close friends and family-members to play one another's games whilst saving their own progress and achievements, via a shared computer.
Join this group if you'd like to sign-up for the beta.

Press release: http://store.steampowered.com/sharing/
Join this group if you'd like to sign-up for the beta.

Press release: http://store.steampowered.com/sharing/