I've been researching this same thing very hard the last couple of weeks since I'm wanting to get a smaller CNC.
So far I like Torchmate, Shopbot, CamMaster, and E-Z router. I saw one of the E-Z routers last week and it's a very nice machine.
Also I think customer support is important. If you feel up to the task for building one yourself (depending on what you're using it for) Fine Line Automation has kits for CNC routers up to 4'x8'.
Hope you find what you're looking for!