There are a few made by Bombshell Babes - look in their Counterblast game line. Think there are a few retro-style walking and hovering robots, and an entire faction for the game - Mekkus?
Hydra miniatues do a line of retro robots.
Several of the resin bitz makers do robot extras - look at Kromlech, Micro Art, Puppets' War, Anvil Industry for interesting pieces. I have some robot legs and tracks to convert other figures.
Actually... Anvil Industry have a nice pack of tracked gun platforms (weapon and tracks separate), UAV-style drones and a pack of humanoid robots.
Pig Iron has some big droids.
The old WotC Star Wars prepaints line has a TON of droids of all shapes and sizes, easily repainted.
Guitar strings are nice and cheap for your own conversion - hit the bits box!
Black Cat Bases made some, but they're long gone - put a Wanted request in the Bazar?
Have a poke round Reaper's site - they seem to have a bit of everything. Maybe the Chronoscope line first?
eM-4 has some cheap mecha-style robots that are human-ish sized for 28mm
Some of the smaller Martian walkers from the All Quiet on the Martian Front line?
GW stuff for Necrons is too distinctive, but their Adeptus Mechanicus line has a couple of tracked servitors and the robotic-looking Sicarian Infiltrators, not to mention a ton of gribbly bits. A lot of their hand weapons are VERY fragile though. And expensive
There are several Tau drone variants, and a kitbash would be a lot of fun.
Ainsty have several in their Spy Fi line.
Warlord Games - their Judge Dredd line?
DALEKS! How could I forget Daleks? The Black Tree Design studio Dr Who range will have a ton of other robots as well - mechanoids,
White Robots.
LOVED Paradroid!