Warp lets you mouse between Spaces in OS X
OS X's Spaces feature is a nice implementation of virtual desktops, but I find it can be awkward to move between spaces sometimes. As much as I'm a keyboard shortcut kind of guy, and enjoy the ability to use the keyboard to switch spaces, often times when I want to switch between spaces my hand is already on my mouse. Yes, you can switch spaces by clicking the spaces icon in the menu bar, but there's a faster, more intuitive way.
Warp is a system preference pane that adds the ability to simply push the mouse against the side of the screen and move to the next Space. There are configuration settings for how long the mouse needs to remain at the edge before it activates, whether to warp the mouse cursor when switching spaces (so that you can continue one smooth movement as if the spaces were actual screens side-by-side), the ability to wrap around spaces, whether or not to warp on edges featuring the dock or menu bar, and whether to require a click at the screen edge to activate Warp.
You kind of have to try it to understand just how it feels to use, and Warp may well require some tweaking to make it agree with your way of working. But once you get it working, you'll wonder how you could ever live without it.