minimalmac:

Dragging and dropping is a great way to get stuff done on your Mac, but DragonDrop makes it even better. DragonDrop lets you set down what you’re dragging, leaving you free to find your destination without worrying about keeping the mouse button held down.

Make sure to click through on the link to check out the video on the site. I’ve seen other similar apps that do the same thing for the same reason, but they are usually some sort of tab or drawer on the side of the screen. This activates with a jiggle of the mouse. It’s a really interesting, and more transparent, approach. 

Update: It appears not to be available yet (still awaiting app store approval). My bad. I’ll repost once available.