While Steam tracks the time you've spent playing individual titles, it doesn't keep track of how much time you've spent on the service overall. For some, that's probably for the best, but just in case you actually want to see that tally, now there's a way.
This handy site lets you enter a Steam ID then wham, your total playing stats are presented, along with some very nice pie charts breaking everything down in picture form.
A word of warning, though: it actually didn't work for me, so if you've got a lot of Steam games, don't freak out if you get a "0 hours played" result.
Steam Profile Analysis [Lambent Stew, thanks everyone who sent this in!]