OS X Mountain Lion: keyboard shortcut to move current window to another space

Solution 1:

You could use sizeUp.

sizeUp allows you to send the window to the next/previous space but also it has keyboard shortcuts for mounting a screen to the left or the right of a screen and sending it to the next/previous monitor.

Solution 2:

Slate might be an option for you:

Slate is a window management application similar to Divvy and SizeUp (except better and free!). Originally written to replace them due to some limitations in how each work, it attempts to overcome them by simply being extremely configurable. As a result, it may be a bit daunting to get configured, but once it is done, the benefit is huge.

Slate currently works on Mac OS X 10.6 and above