At home, I put my macbook pro on the left of my widescreen monitor. At work, I put my macbook pro on the right of my 19" monitor. My mac automatically recognizes not only that I have hot-swapped a DVI monitor, but also which monitor I have plugged in, and where I last positioned that monitor. For total icing on the cake, I can configure the relative virtual vertical position of the two monitors, so that I can move the cursor straight across from the top of my laptop screen to the middle of the external display.
I love you, OS X. You complete me.