Competition

Google Apps

Confluence

http://www.atlassian.com/software/confluence/

MediaWiki

http://www.mediawiki.org/wiki/MediaWiki

switch

Sharepoint

http://sharepoint.microsoft.com/Pages/Default.aspx

file systems

Drupal

http://drupal.org/

SocialText

http://www.socialtext.com/

Mindtouch

http://www.mindtouch.com/

XWiki
MindTouch
Twiki
FOSWiki
MediaWiki
Drupal
Microsoft SharePoint
Lotus Connections
BlueKiwi
Jive Software
SocialText
Atlassian Confluence
Telligent
Jalios
Alfresco