DisplayFusion - DisplayFusion will make your multi-monitor life much easier.

Yoink - Yoink is a website that makes it easier to drag and drop images and text from one screen to another. It's a straightforward site with help along the way if you aren't sure about dragging and how to place your content.

Mizage Divvy - Divvy is an entirely new way of managing your workspace.

Dropover - Mac app for easier drag & drop

WindowGrid - Quickly and easily layout windows on a dynamic grid using just the mouse.

Dropzone - If you want your file uploads to work even without JavaScript, you can include an element with the class fallback that dropzone will remove if the browser is supported.