OS X: Macs still don't have the ability to cut and paste folders and files in the Finder, but Cut with Drag & Drop brings an easy way to move your files without copying them first.
While you could always open up both folders and then drag the files yourself, Cut with Drag & Drop makes it easy to do so from whatever folder you're in right now. Just open it up, drag your files to its dock icon to cut them, and then drag the destination folder to the dock icon to paste the files. It's not as easy as just pressing Cmd+X (which the $8 MoveAddict will let you do), but Cut with Drag & Drop is free and will do the trick in a pinch if you don't want to open multiple Finder windows.
Cut with Drag & Drop is a free download for OS X, available in the Mac App Store.
Update: It seems OS X Lion does have a cut and paste function built-in, it just does things a little differently: Press Cmd+C to copy as normal, then Cmd+Option+V to paste. That's it! Snow Leopard holdouts can still give this app a shot, though.
Cut with Drag *& Drop | Mac App Store