Cheap hardware.

regular keyboard for main machine
wireless keyboard controlling 4 machines
1 mouse for each of the 5 machines

I'm using boxes made from old parts for 3 machines(quite literally junk), my decent machine that I use for every day use and a laptop. I've done 2 or 3 characters at a time in the past usually just for leveling or doing something I couldn't do solo. Having had all the machines sitting around it took me very little to setup a 5 box system. I went and bought a cheap wireless keyboard and ordered 3 replacement usb adapters. This works extremely well, there are occasional missed keystrokes but the few hundred bucks I'm saving makes it worth occasionally having to tap the key twice. I do that regularly when I'm not hitting a key binded to a cast sequence.

My reason for using a pure hardware solution is the same as previous posters. I'm not willing to risk losing the time and effort put into leveling and gearing my characters when blizzard decides one of these 3rd party programs needs to be added to wardens watch list.