not being a canon junkie or sheep....
i cant really say, but the best deals seem to be the d10/20/60. I dont think the rebel really holds up well enough to trust it used. i supposed a canonite will be more than happy to try to educate me.
in the nikon world you have the d100 and d70,also noting the fuji S2 has fallen to the under $500 mark. you can find the kodak slr/n falling in price as well, its full frame.
for sigma its really the sd10.
for oly mosy folks say the e1 but the e300 and 500's are cheap as well
not sure on pentax
the big thing is glass, the cost of oly glass gives me sticker shock, but oly folks says its cheap. the sigma can only use sigma glass, but there are a lot of mods out there. nikon glass is better than canon, even those canon folks live in egypt along the river denial, thats why it cost more than canon glass.
with canon you have a plethera of bodies and sensor format sizes. with nikon you get the ability to use different sensor types, such as fuji (oohh those colors) and kodak (color and iso of 6).
if the nikon bodies i really feel the fuji S2 is the best of the lot due to the ease of use and the colors, then the D100 and then the D70
the old minolta's are out there cheap now that sony took that line over.