Stars!, now that is a game that has cost me many hours of my life. The game is just so addictive!

Also, Civ 2, that is another big time stealer for me. While I think that Civ 4 & 5 are outstanding games as well, Civ 2 manages to hit a sweet spot when it comes to complexity, it is a game that I can play without needing to give my full attention. If I want to play something complex I'll usually turn to something along the line of Victoria: An empire under the sun or Steel Panthers.
Steam has not always tracked how much time you've spent playing though.

were you in offline mode?

Yup. It never stores playtimes when you are in offline mode, I know that for sure. It says I finished FEAR2 in 10 minutes and got to half of Oblivion in 6.

Sometimes, Steam refuses to store playtime just because it can.

This happens too. I believe sometimes it happens when you loose connection, like steam servers crash or something, than when it's restored, it doesn't upload your new playtime, it restores the old one.

Happened quite a few times for me.

That's the weird thing. I hadn't actually played the game in a while. So I would have thought that if I lost that one to a crash, I'd have lost all the play time on the more recent titles too.

It's not that I'm fussed about it or anything. It's just my way of pointing out how there's really no point putting any stock in what Steam has to say about the stats it has recorded.

That's probably why they haven't released any sales info? Maybe they struggle with that too? :D

If we're just talking about Steam then I've wracked up +500 hours with the Total War series. Most of that is with Rome but there's a sizable chunk with Empire and Napolean.

Steam record keeping:

Does not work in offline
Does not work when you lose internet connection mid game. Even if you regain it during the same gameplay it will probably not record the time properly (same goes for achievements. only in online)
Steam servers sometimes reset themselves. rare. I did lose 2.5 hour in FUEL because steam went bonkers for an hour or so, and when it came back it showed my old timeplay not the most recent.
Game must be started through steam and closed properly. If it crashes if you CAD it, or never properly exit, as well exit steam correctly, timeplay will not be recorded correctly. Can happen when your system kills steam instead of shutting it correctly when shutting your computer. Does not happen all the time but if steam did not upload the time correctly before it was forced by the system to close, timeplay might be lost.
Steam started to record timeplay around September 2008.