Mac OS X has a nifty text feature where you can hit Command-E, and your selection will be placed into a find buffer. Hitting Command-G and Shift-Command-G will find the next or previous instance of your selection.
I find this really useful when poking around in code. For instance if I want to go look somewhere else in a file, I will select a unique chunk of code, and stash it in the find buffer. I go about my business and then as soon as I need to go back, I just hit Command-G. It’s one of those basic little things that really makes working on a Mac quite enjoyable.
Any other good keyboard shortcuts people tend to miss?