A list of miniatures purveyors on the internet, with annotations, organziation, and additional pictures where possible.
This list is meant to deal with merchants who sell individual or bundled miniatures. Excluded are large, well known game companies (such as GW or Corvus Belli) about whom there are undoubtedly far better sources of information.
Organized by seller for now, but if the list gets too long I may also break it up by scale.
Scotia Grendel produces several lines of fantasy and scifi miniatures. The main ranges fall into 28mm and "micro" which includes very small tanks and planes. The 28mm range has some rich variety, and a lot of very strong themes. The metal miniatures are of excellent quality, though the plastic Viridians where a bit hard to assemble, and the resin models where very difficult to work with. The resin models I got (the urban tank and the Nexus tracked tank) where cast as giant, heavy blocks. I was sort of picturing sprues, not sure why though.
Made of both new models and old Ral Partha metal sculpts. They're still your source for Battletech models, if you play classic battletech. If you're just in the market for giant 6mm-scale robots (and honestly, many of them are humanoid enough to pass as 25mm or 28mm androids) they are a bit pricy but have a very wide variety of sculpts. Sometimes the quality of the sculpts leaves a bit to be desired - for example, the mining mech is assembled with tiny cylendrical extenders which really, really don't want to stick to the body or arms. They've also got fantasy miniatures (Ral Partha did D&D minis for a while) and some spaceships for battletech, and some BSG ripoffs. Some of the aerospace craft for Battletech would work fine in a spacefighter-scale game (ie Cold Death).
Almost too legit to include on this list. You'll see them in game shops because they make tons of models that fit well into people's RPGs. Their CAV line features some large (6mm going on 10mm I'd say) tanks, robots, and infantry for your 6mm wargaming needs. They also have a line of fantasy wargaming minis that I assume are meant to be 28mm, but they tend to run a bit larger than, say, Space Marines. The materials used deserve perhaps special mention. In addition to the usual pewter they also have two types of plastic-one fairly regular if a bit soft plastic (which I'll call gray plastic) and a soft, slightly porous white plastic they've branded "bones." The gray plastic is slightly nicer to work with than the hard plastic you'd find on a sprue, but often has difficult-to-remove mould lines and may come slightly bent (I've seen blister packs of this not-A10-not-warthog with severely bent wings.) Bones, on the other hand, is an absolute joy to work with. It can be bent by heating, knives will cut very cleanly through it, and it is light enough that dropping a model won't usually harm it. Only drawback I can see is that most of the figures are single-piece allowing for little customization (this applies to all of the materials, but it's most easily corrected with Bones where you can just slice them up and glue them back together, or heat and bend to pose.
They make Full Thrust and, fittingly, a line of themed spaceships. They've also got a couple of different scales of terrestrial scifi miniatures.
Unfortunately, they often lack pictures.
Brigade models also sells lines of themed spaceships as well as other misc. metal models.
In addition to many historical figures, there is a hodge-podge of scifi miniatures including Daleks from Dr Who, Star Trek style ships, hovertanks, Dune worms, and probably others that I don't recognize.
Tons of scifi spaceships, including several different star trek lines and even some Mass Effect. Another notable feature is that they have metal and/or resin casts of a few Irrational Designs ships, which are usually on shapeways. Very nice designs but shapeways is high priced and the 3d print quality is a bit rough at tabletop scale.
Mostly historical miniatures, but they also have a fantasy line.