I still recommend Reflection X over just about anything free. It just seems to work better and if you have to use Windows you might has well have some good X-Windows/Windows interactions.
It at least might save you from throwing your mouse across the room.
That said though, VNC and Cygwin's Xfree make reasonable alternatives. For long time/hard core type work though I would really recommend a commericial solution.
Plus there is nothing quite like having somebody spend more money and a Windows Client box just to turn it into a usable X-Terminal :)
