It's easier to just link to this:
That's not everything... some items, or companies, are so close to stomping up and down on Hasbro's IP that US companies are afraid to touch them... but it's most of them, i think.
Some companies are higher rated than others.... Fansproject are probably favourites.
iGear.... shedding their KO image a little... faith leader was widely slated for low quality,iirc,...but they've had people hanging for months on Ratchet and Ironhide, and there is something just very slightly off about their coneheads.
Xorvengen were pretty much slammed for their G.I.Joe VS Transformers themed Rolling Thunder Optimus..(lousy quality apparently) . but have a Powermaster Prime themed item in the works....
BTS meanwhile pushed out a few new tapes and a Soundwave based on Guidio Guidi's dictaphone inspired design..... that met with a rather poor response, tolerance issues, i think, and last i looked BBTS coudln't give the thing away.
Beyond that.......some accessory companies like Dr Wu are just copying people's work from Shapeways
If you are looking at shapeways, particularly look at what fakebusker83 / ariel lemon is turning out.
Not going to go any further than that for the moment... 3rd party is such a wide, varied, area now.