Tumblr is a free, simplified blogging service that facilitates the creation of Tumblelogs (popular examples include Projectionist & Anarchaia), which are essentially streamlined versions of standard blogs.
Tumblr is great if you want to setup a quick and easy link blog or photo blog. You can even have Tumblr import RSS feeds that automatically post to your blog, as well as mapping your own domain to a Tumblr blog if you want.
Eric Rice calls Tumblr a “hybrid site between twitter and wordpress” and Jason Schupp says “What’s old is new again”. One thing is for sure, Tumblr part a whole new wave of services that are helping people get back to the basics of blogging.