Depends which system you shoot of course, but probably the best value to be had these days is a used 5D. Here in Canada they can be had for $1200 sans grip or $1500 with the vertical grip. Somewhat cheaper in the US.

Unless you have considerable amount of non-Canon lenses, the original 5D is definitely a steal, if you can find one. The next closest price point would be Canon 5D MkII or Nikon D700. They go for around 3 thousand new, so used condition will be somewhat cheaper. I'd say those two are only justified if you absolutely need a specific improvement, such as larger images or higher frame rate.