Since it's coming up one Christmas, Hanukkah and whatnot season, I thought I'd toss out some little OS X nuggets I've discovered over the years. These aren't full-fledged features per se, more like little stocking stuffers that not everyone is aware of:
Control-Click any text in cocoa apps for a quick dictionary/thesaurus definition. Just choose "Look Up in Dictionary" from the resulting menu and you get a mini popup dictionary. Very handy for when I want to look up a quick synonym.
Command-Click filename in window title bar to get the full path. This is useful if you have more than one partition or you're just disorganized in general.
Command-Click and drag menubar items to change their order. For the proper feng shui.
Shift-Click the Force Quit menu item to force quit current application. This is a nice easy way to relaunch the Finder.
Command-Option-Control-8 reverses or inverts screen colors. This is great if you're reading a black-background web page with white text and it's killing your eyes. Just hit the above key combo and you have black text on a white background. Also this can be good for dimming your screen since most of your screen is usually white or light colors. Like if you want to use your laptop in bed but you don't want to wake your partner after a particularly rigorous lovemaking session, just hit the keys and you have a nice dark screen that still retains contrast.
Also, dragging your home folder into the dock makes the Finder much more usable.
Now if only I could find a way to remove the existential taunting from a blinking cursor.