This OS X versus Windows XP site contains an exhaustive, extensively illustrated 100-topic comparison of these two operating systems. The author tries to be objective, which is admirable, but the extremely detailed comparison is worth reading mostly because it highlights a lot of subtle design differences. For example, this little feature of the OS X login dialog:

Furthermore, if you type your password wrong, the login window wiggles! This simple, elegant and unintimidating feedback lets users know that they need to try again. The down side to a wiggling login screen is that it may be insufficient to notify a novice user that they entered the wrong password. A written confirmation that the password was not accepted would help novices.

If you like comparing GUIs, you'll probably enjoy the GUIdebook website, too. It's a great source of current and historic GUI screenshots. I first stumbled across this site while searching for old Excel screenshots. I didn't find any, but I did find this incredibly neat visual comparison of every Excel splashscreen since Version 1.01 on the GUIdbook site. Who knew splashscreens could be so.. cool?