A danbooru-style image board, designed to be easier to install, run, and extend.
Requires a standard LAMP stack as provided by any normal web host. It can also use PostgreSQL as a database, which is recommended for high-load sites.
This is probably my most popular project, with a team of 10 coders, hundreds of installations, and thousands of users online at once.
Official website: http://code.shishnet.org/shimmie2/