Hacker News - Hacker News is a social news website focusing on computer science and entrepreneurship. It is run by Paul Graham's investment fund and startup incubator, Y Combinator.
Typehut - Super simple publishing platform
Reddit - Reddit gives you the best of the internet in one place. Get a constantly updating feed of breaking news, fun stories, pics, memes, and videos just for you.
The New WordPress.com - Publishing platform, reinvented, open source w/ new Mac app
Telescope - Telescope is an open-source app that lets you easily set up your own online community.
Caramella - A clean, modern blogging platform without the fuss