Best open-source Wiki

CitadelCitadel is a collaboration suite (messaging and groupware). It provides support for Email, Calendaring/Scheduling, Address books, Bulletin boards, Mailing List Server, Instant Messaging, Wiki, Multiple domain support. TWiki - Wiki and Web 2.0 Application PlatformTWiki is a flexible, powerful, and easy to use enterprise wiki, enterprise collaboration platform, and web application platform.