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).