The reason Samsung is so popular is that it does not come with those silly, unnecessary heatspreaders most other companies force on you. Also as far as I know all Samsung RAM is actually manufactured by Samsung. Crucial is mostly ( 90%) Micron made since they are basically the same company. Corsair is just a rebrander and they use modules made by Qimonda, Elpida, Micron, Powerchip, Hynix and even Samsung. Probably a few others as well.
Here is a pretty incomplete list but it's the best one I know of as to who makes what.