Mailing Lists (best place to start for groups that are unstructured)

• mailman (chosen product of ibiblio)
• mailinglists.org/google groups/yahoo groups
• archive services (default mailman archives are not very searchable)
• MHonArch

Wikis (check out the comparison list on wikipedia.com or wikimatrix.org)

• mediawiki (open source software that wikipedia runs on) Dreamhost
• pbwiki (can cost $$$ for easier implementation)
• dokuwiki
• confluence ($$$ - possibly free licenses for open source projects)
• socialtext.com
• jotspot.com (acquired by Google)

Blogs

• planet server planetplanet.org (aggregator for blogs)
• wordpress (cms lite)
• textpattern (cms lite, semantic)
• lyceum
• wordpressMU
• apache roller
• six apart (livejournal, vox, typepad)
• twitter
• tumblr
• pownce
• soapblox

Content Management Systems

• drupal (decent user base, crm-friendly)
• plone (platform)
• joomla/mambo (crm-friendly)
• bryght.org (hosted drupal installations)
• wordpress (cms lite)
• textpattern (cms lite, semantic)
• expression engine

Social Networks

• ning.com
• people aggregator
• facebook
• myspace
• linkedin
• flickr

Social Bookmarking Tools

• del.icio.us
• foxcloud (firefox) and fox
• digg
• magnolia
• connotea

Shared Calendars

• google calendar
• 30 boxes
• upcoming (yahoo)
• bedework

To Do Lists

• remember the milk
• 37signals.com

Instant Messaging

• twitter
• gtalk
• jabber
• adium
• pidgin
• meebo (website)
• IRC
• gabbly chat
• campfire

Voice

• grand central (bought by Google)
• freeconferencecall.com
• gizmo project (skype alternative)
• skype
• asterisk (open source PBX software) --- featuretel or cerient | Triangle Open Source Lab (trixbox and asterisk now)

CRM

• civicrm
• sugarcrm
• zoho

Project/Document Management and Version Control

• google docs
• launchpad
• basecamp
• active collab
• zoho
• trac