This is the killer feature in Windows 7 as far as I'm concerned. This and the searchable Start Menu.

It's simple, but I use it dozens of times a day. It's quick. It's effective. It's listed in the title of this blog post, completely undermining any attempted suspense. It is, of course, the wonder of ICT Tip 037 - Aero Snap.doc!

A lot of people have no idea that this exists, despite it being useful for nearly everyone. Send it out. Tell them. Share the joy of knowledge. Just don't charge them money