Anvil does fantastic multi-part resin kits, easily comparable in detail to Games Workshop. They're also in the process of producing models specifically for a narrative driven wargame. If you're on the Second Sphere Member group, you'd have seen me sharing the pictures of these models
Zealots own models are equally detailed, whilst they also sell products from other producers. They have a great range of terrain, both for Surface battlefields and dungeon/sewer tunnels. They also have a few nnone wargame specific miscilaineous items, which would make for excellent painting practice pieces.
Mantic specialise in medium to high detailed plastic kits at reasonable prices. they have several unique and different game systems under their belt and are regularly releasing something completely brand new, or a fantastic additon to one of their longer running products.
I unfortunately don't know how good Privateer Press, Infinity, or Dreamforge Games are, so you'd have to ask one of their customers for an opinion
Puppets War create and sell highly unique and interesting Resin kits.
Mad Robot Miniatures is the place to go to give your Imperial Guard/human armies a bit of extra flavour and character.
Secret Weapon Miniatures and Zinge Industries offer some excellently detail resin conversion parts.
Scotia Grendal offers a fair number of game systems and the models to play them. I've recent purchased the Rulebook and a Forcebook for Void 1.1, and they're fantastic quality. A large range of older metal models, and newer plastic sets produced for independant companies.
Thats just a general idea I have for each of the companies I mentioned previously