There are many, many, many other such boards, but none of them are very popular. Unless you like porn.

Yes, I like porn.

4chan started as a clone of the Japanese Futaba imageboard, which was itself a variation on the premise of the (also Japanese) 2ch text board.

Other people have since made sites that, as I understand them, boil down to "4chan for people that speak language X" (eg Krautchan (German), Ylilauta (Finnish)) which are themselves fairly popular and influential.

Beyond that, there are many small and interesting English-language boards that aren't "just for porn," but as krapp said, most aren't very popular, but tend to have tightly knit communities (as strange as that sounds, talking about an anonymous board) that I'd feel wrong drawing unwanted attention to.