Yesterday, Ars Technica published an exhaustive look at Steam sales figures. A day later and some slight adjustments have had to be made, thanks to some corrections to their data (it's not a big deal), but something has been added to the report. A list of the top 100 games on Steam.
Like the rest of the report, since it's mined from user data (not released by Valve as official records), there's a margin of error to take into account. Like the rest of the report, though, that margin is small enough for us to all shrug, not really mind and be thankful we've got a mostly accurate report, since that's better than no report at all.
The top 20 is below. As you can see, it's dominated by free-to-play titles, especially Valve's own experiments in that field, though retail heavy-hitters like Skyrim and Civ V are right up there.
You need to check out the full chart though, since there are a few surprises near the end.
The 100 most played games on Steam [Ars Technica]