Hard to find and expensive, but we found it at work: the kvm switch that actually works. For us, that is. A environment of almost all x86 hardware with a mix of Linux and Windows as operating system.
Our older kvm switches either had no idea about this 'network' thing or (the Avocent Switchview IP) an approximation: the only working network client was the activex component served by its own webserver. Handy when you're at home using some ssh forwards and without internet explorer (or windows). No access. The 'about' box on that stuff said it was based on vnc but no vnc client could work with it.But now we found the Adderview CATx IP. Cat-5 cable from the video + keyboard + mouse (usb) adapter to the adderview. And any recent vnc client will work with it, even over portforwarding via ssh links. The only downside is that the outgoing vga and keyboard connectors are on the back of the unit so it's a bit of a nuisance to plug in the 'crash-cart' (a table with wheels with a screen, keyboard and mouse on it) we use. Fixed this by using vga and usb extension cable.