Made something really similar to apple's recently unveiled "universal control," but from Windows -> Mac. Lets me have the mouse wander off the side of the screen if "pushed hard" and it "pops out" on the mac. It has some issues with modifiers sticking and it's hard-coded to my network IPs and screen resolutions, PLUS it requires an unrelated tool I made to actually build the thing; it's on my "open source someday" list but it's not very other-programmer-friendly.

Made lots of other things but that's probably my fave and the strongest one for productivity.