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.