Wiki (collaboration tools) Roundup

This is a reference post for wikis (and other collaboration tools) that I need to check out:

• Basecamp (already using)
• Confluence
• Central Desktop
• Jotspot [Hosted]
• Social Text [Hosted]
• Stikipad [Hosted]
• WikiSpaces [Hosted]
• PBwiki [Hosted]
• Zimbra
• Perspective [.Net]
• FlexWiki [.Net]
• MediaWiki [PHP]
• DokuWiki [PHP]
• TikiWiki [PHP]
• PmWiki [PHP]
• Twiki [Perl]
• Kwiki [Perl]
• trac [Python]
• Zwiki [Python]

Update:

WikiMatrix looks at these too.