Alternative products compatible with LEGO
there are at least:


KRE-O (not sure if this is compatible in size, but it looks exactly like LEGO)
Enlighten
Best-Lock
Blockmen (out of production)
Cobi (wich you already listed)
SuperBlox
iDol
Kazi (looks like poor quality)
Sluban (out of production)
Mega Blocks
Super Blocks (out of production)
Custom-Crazy
Sidan Toys
Arealight
Brickforge
Christo
BrickWarriors
Oxford
Little Armory
Nanoblocks (not sure, but looks smaller than LEGO, so this might not be compatible)


Note that some of this only produce custom accessories for minifigs, very small sets or exact copies of the official sets, no own really "big" sets like LEGO does