Mac OS X: If you've ever had so many windows open that dragging and dropping a file between them becomes impossible, Yoink makes the process simpler by adding a small drawer to the side of your screen.
If you're on an 11-inch MacBook Air (or heck, even a 13-inch laptop), dragging files between windows can sometimes get difficult. In many cases, you can hold the files over the destination's dock icon, but if you're dragging files to and from the desktop, or dragging a group of files to two different places, it still becomes a big pain. Yoink adds a small sliding drawer to the side of your screen the appears when you drag items in the Finder. Just drag an item into Yoink's drawer, open up the destination window, and drag them out. It really makes the whole process much less stressful.
Yoink is a $2.99 download for Mac OS X only. If you're sceptical on the price, you can try it free for 15 days from Yoink's homepage.