Might need a bit more info than "space mariney", but it's a fun game. How about these:
The CPP 209 (Robocop ED-209 Homage) from Mercs: Recon (PVC, not hard plastic):
Leviathans (28mm & 15mm) From Dreamforge:
There's also the OOP AT43 Walkers.
Lots of walkers from Dust Tactics.
Star Wars AT-ST & AT-AT.
There are the Russian "Robogear" walkers (Gryphon, Salamander, T-Rex).
EDIT Predatorpt beat me to it!Dreampod 9 now makes plastic kits for their Heavy Gear Blitz line (12mm).
That's all I can think of in plastic. There are a ton in resin though.