Blogger.com - Majority of the bloggers started of with Blogger. A great starting site to get a taste of blogging, very easy to use. Has almost everything you need from widgets to template designing and modification. The entries are easily indexed by Google search engine because it's owned by Google.
WordPress.com - The free and easy way to blog with Wordpress without the need of technical blog installation knowledge. It has most of the latest features but with limited access over the modification of the preset template. As a result, advertisements from external parties aren't allowed. Nevertheless, it offers inspiring writers to pen down their words professionally.
Tumblr.com - Probably the easiest way to blog. A platform with a focus on allowing media-rich posts. Suitable for those who love to share photos, videos and links without the need to lengthy words. Allows others to re-blog a particular post as well, that's the unique feature of Tumblr.
Posterous.com - Labeled themselves as the easiest way to share photos, collaborate with others, share new ideas, create a travel journal, share passions and share videos. The unique feature about Posterous is that you don't need to sign up, know any code or even know how to do anything. Just send an email to set up your own Posterous blog and start sharing your ideas and media.