From what perspective are you coming to a Mac? Are you a power user in the past on another platform like Windows or Linux, or do you consider yourself a more novice user?
You might like X vs. XP -- it's sort of a shootout between OS X and XP, but more to the point, it really does a great job of walking through the things one does most commonly on a computer and showing how they are done in Windows vs. OS X. So it helped me a lot when I switched.
Mmmm, some other sites that I use a lot....
http://versiontracker.com -- is great for finding any app by name or keyword http://macosxhints.com -- I don't exactly type this in the address bar a lot, but it's an amazing repository of solutions to common (and not-so-common) problems
There are also a bunch that depend to some extent on what apps you use. For instance, http://pimpmysafari.com is great if you use Safari. 🙂
iusethis.com, helps you find the popular third party apps quickly with their top apps section (although they include the some of the Apple apps that you already have on your computer).