Mantic Games tabletop wargame "Warpath" provides gamers not only with alternate models for IG, and Ork, but an entire alternate game which includes races overlooked or forgotten by GW. Veer-myn are space Sci-Fi ratmen and Forge Fathers are Sci-Fi dwarves. At $75 US for a 43 model starter army these models are priced right for anyone looking to add a custom feel to their GW army or looking for a more affordable alternative game.
Pig Iron Productions produces near future and Sci-Fi 28mm miniatures and miniatures bits. Whether you're looking for completely unique miniatures or just some bits to add some flare to your minis Pig Iron has a solution for you.
Victoria Miniatures has a wide selection of highly detailed guard alternatives. They have their own Penal Legion Squad for example which is truly breathtaking. In addition to the penal squad they have a line they call "Border World Rangers" which would make a great Tanith substitute.
Maxmini produces a wide variety of bits for any army you play. They also have some complete miniatures including the above pictured Ogryn alternatives "Space Ogres".
Puppets war produces bits and many alternative models for the 28mm wargaming community including many vehicles.
Secret Weapon Miniatures offers an amazing selection of bases and weathering pigments. I personally use their line of weathering pigments on my models. In addition to pigments, they also sell some great bits and a few models like the badass tank pictured above.
Anvil Industry produces some sweet weapons and other bits for 28mm miniatures. I have a few of their heavy weapons and a bionic legs kit. The also produce some unique miniatures like those shown above.
Brother Vinni's produces high quality custom miniatures and even do commission work. They have a few alternative IG models including the above pictured commissar. I'm currently using this model as my Lord Commissar and will be using him in the future for General-Commissar Gaunt.
Chapter House Studios produces some great bits for use with IG including the above pictured wheeled Chimera conversion kit and a female guard unit.