I’m always kind of jealous that I own the games already when this happens.
Anyone who’s passing up on the chance to try out Steam while getting the chance at playing one of the best games ever is clearly not in possession of a full tea set.
@Crytiqal
As LyndonL said you can add games that were not purchased through Steam manually to your Steam library but the functionality is pretty much limited to launcher only. That said some games allow you to register the key on Steam at which point they’ll integrate right in as if you bought them there (I did this with my old copy of Prey and now never need to go hunt for the DVD).
As for friends and what steam does. I guess apart from the obvious distribution platform there are social tools. You should be able to add friends that are also on steam but there is no cross platform stuff so email/msn/live/xfire usernames won’t help.
Steam currently peaks at around 2.3 million users online. Leave it running in the background like another instant messenger program and you can be invited to games by your friends, chat etc. They’re still building on that stuff though so while I appreciate it’s not hard to knock more of the obvious stuff they’ve yet to do.
Give it a try, install Portal. If you’re gonna play Brink, join the Brink group etc.