i just found a nice OS X feature by accident. Hold down command and option (you’ll see the cursor change to a crosshair) then drag-select some text in a web textarea. It will select a rectangular area, just like a cmd-opt-4 screen grab, but for text. Not sure if this works in any other contexts.
Another neat feature I read about recently is very cute: place your mouse over a word (in pretty much any app), then hit command-control-d, and it will pop-up a neat little word definition panel.