I've used the software Synergy [1] in the past as a software based KVM which has the advantage of being cross-platform with linux and Windows. It works fairly well but some of the features that Apple is promising with Universal Control are ones that Symless has struggled to deliver in their product, probably due to the fact that they don't control the entire stack like Apple does. They've promised drag and drop, copy and paste, and other features between systems but it seems that they've stopped advertising them as upcoming features.
Universal Control stood out to me as one of the more interesting ones of the WWDC keynote and one that I could see myself using, but am a tad skeptical it'll deliver on its promise fully. Apple's other continuity features are remarkably useful and great when they work, but in the cases that they don't (which while not normal, isn't uncommon enough to say they're a non-issue) it can be a real pain. This sort of feature needs to be rock-solid if you intend to integrate it into your personal workflow.