b2evolution is probably the most comprehensive blog engine you could find, with an unique set of features including:
* Simple installation
* Multiple blog support
* Multiple user support
* Multiple skin support
* Localized in many languages
* Multilingual blogging
* Web standards compliance with integrated XHTML checker
* Clean search engine friendly permalinks
* Advanced user permission management
* Private/protected posts
* Open/closed/disabled comments
* Trackback & feedback
* Integrated hitlogs and stats
* Robust antispam features (referrer & comments)
* Plug-in architecture
* Advanced rendering plug-ins (Textile, Auto-P, BBcode... )

http://b2evolution.net