Since Typo has been in existence since March 2005, it is likely the oldest blogging platform in Rails. It has a full set of features you would expect from such an engine, which include powerful SEO capabilities, full themes, and plug-in extensions.

the tiniest blog-engine in Oz.

Add a blog to your Rails application in minutes with this mountable Rails Engine

An embeddable file-backed content management system.

Serious is a simple, file-driven blog engine inspired by toto and driven by sinatra with an emphasis on easy setup

Because Blogojevich would be too tough to remember. It's a pluggable blogging system for Rails 3.

A really straightforward open source Ruby on Rails blog engine designed for integration with Refinery CMS.

PiccoBlog is a simple Ruby on Rails markdown blog engine.