A couple I've found:

• The standard mouse driver is the WORST EVER. I've never used a system with such horrible mouse acceleration, and no way to turn it off. FIX: http://www.usboverdrive.com/ - a custom USB driver that give much smoother mouse movement, and lets you configure the buttons better. Also lets you disable the horrible acceleration on the scroll wheel.
• I've seen some people complain that they can't put Dashboard widgets on the desktop. You can, if you turn on development mode: http://developer.apple.com/technotes/tn2005/tn2139.html#TNTAG13
• A similar little command makes Safari a whole lot more useful to developers, turning on the debug menu which contains lots of goodies: http://developer.apple.com/internet/safari/faq.html#anchor14
• For those who like the command line, there are a lot of tools to access Mac OS specific stuff. I've had some fun with osascript, which can issue AppleScript commands to control apps.
• I had some trouble getting screen on remote machines and the Terminal to get along, until I changed the terminal type in the preferences to "rxvt".

Finally, an unsolved problem: I have been completely unable to find an even halfway decent image viewer for Mac OS X. I want something with the basic functionality of ACDSee on Windows, like the automatic browsing through directories of images by just clicking on a single image in the directory. Also, something that is decently fast, and does image decoding in a separate thread so you don't have to wait for each image to load when rushing through a long list of them. There's ACDSee for Mac OS, but it just fails it. I am considering writing my own.