Once again, The Steam Review has dug up some tasty information regarding the future of Steam. What might that information be? Well, if you haven't read the news topic yet, allow me to repeat myself. Guest passes may be headed to a Steam client near you.
Regards to Andy S, who spotted a series of new strings added to Steam’s interface in the recent platform update. “Guest Passes” are on the way: you’ll be able to hand out free copies of participating multi-player games to your friends, through Friends or via e-mail, for them to play online with you.
Here is a list of what TSR believes will be the case when it comes to these passes.
There are only so many guest passes per game

Passes can only be held in receipt by one account at a time

Passes are returned to you after each use and can be resent

You can only send someone a guest pass once every 24 hours

An account cannot receive a guest pass for the same game twice

Guest passes “expire” after an indefinite amount of time (minutes, hours and days are mentioned)
Of course nothing has been "officially" announced just yet, but you can always check out the details of this yourself by browsing the steamui_english.txt file in your own Steam directory.