I'm surprised how many people don't seem to use Snap.
I use this constantly.
I open my browser and Word side by side and drag text back and forth.
I open two Windows Explorer windows side be side and drag stuff back and forth with out having to switch from drive to drive.
I open my E-Mail client side by side with Word and with Chrome.
I run Photoshop side by side with other programs etc. the list goes on.
I think this is the most useful new feature in Windows 7, I'd never want to be without it again.