WordPress
WordPress is a popular blogging platform. It features free themes made by pro designers, easy analytics, and well developed plugins to let you customize your site. It has become popular as it is an easy, out-of-the-box blogging platform for those new to site design, but also features full customizability for experts.
Wordpress.com is built on the open source blogging platform, wordpress.org
Blogs are available for free, with services such as unlimited users and extra storage available as premium services.