Lifestream plugin for WordPress that gives you a tumblr-esque activity stream aggregated from multiple locations. Slick.
Ping.fm allows you to update a bunch of social networks in one go. Post to ping.fm and have it passed on to Twitter, Facebook, plurk, myspace, linkedin, plaxo, bebo, etc, etc.
gelato cms is described as a tumblelog cms. It's based around the tumbr concept, you can easily post snippets. Various themes and plugins are available.