mewch.net
Mewch.net is an imageboard I am currently a Global Volunteer on. It is much like 8chan in that users can create their own boards and manage them how they like. mewch.net can be thought of as a successor to ChanPink, which was administrated by the same person but later shut-down. A liveposting implementation is currently avaliable, and is being enhanced to look prettier. It is powered by StephenLynx's LynxChan software, which is 100% open-source. mewch.net's front-end is open source as well, and can be found at Github. I highly recommend giving mewch a try!
8ch.net
8chan is an imageboard dedicated to user freedom. It allows for the quick and easy creation of imageboards, even by those with no programming experience. Although it has suffered (and continues to suffer) from multiple software-based issues, it continues to hold likely the largest English-speaking userbase outside of 4chan.org. Some unique and interesting boards include /late/, /tech/, /co/, /r9k/, and /newsplus/.