Key Clone - An AutoHotKey AND Multibox style program
http://solidice.com/keyclone/
Never tried it personally, closed source, costs money (all 3 are the opposite of AutoHotKey/Multibox, available here for free) - but purports to emulate Vetras in software. Quite a few users seem to report it works ok. Put it here b/c some may find it useful, even though in my opinion there are better software methods and even better hardware methods.
"Licenses are tied to each machine and a new one must be purchased for each machine. " - Still quite a bit cheaper than Vetras - may still have drawbacks.
"extended keys (alt, ctrl, and shift) are currently not working across machines." - The developer is working on this.
"only tested on WinXP" - Would be nice if it worked in linux but not likely anytime soon.
Edit: The developer posts here and is working to improve the package. Seems like quite a bargain for those who want to go the software route and avoid AHK. $50 vs $1000+ is quite the savings. I am willing to give this tenative positive feedback.
It also allows you to control clients across the internet (?) and through your LAN, as well as on your actual PC... givign you a great deal of flexibility if you are growing your setup over time rather than building it from scratch.