miniBB is a flat, linear, highly customizable BB. It features MySQL, PostgreSQL, and MSSQL support, table name definitions, color skins, language packs, HTML templates, access control, smilies, search, basic BB codes, member titles and ranks, moderators, email notification, statistics, encrypted cookie logging, IP and user ID tracking and bans, spam protection, "sticky" topics, a powerful admin panel, and more.
Byteflow is a blog engine written on Django. It features Markdown and typogrify for posts and comments, threaded comments, automated registration for a user after comment posting (no anonymous users), registration approval/email changing/profile editing, tags (and filtering posts by tag), the ability to have translated names for tags, archives by day, month, and year, Atom and RSS Feeds for posts, comments, comments for a specified post, posts within a specified tag, and OpenID support. It supports multiple OpenID URLs attached to a single user, a Wordpress importer, Gravatar, an optional WYSIWYG editor in admin parts (for posts, not for comments), and LiveJournal crossposting.