Web conferencing services (Webinars) and software allow multiple people to have a virtual meeting over the web. It can simply be a voice conference call over the web using Voice over IP (VoIP), or a graphical presentation where everyone can see the moderator's power point presentation to collaborative applications where everyone can markup a document.
Also referred to as group video chat or video conferencing

These go beyond services like instant messaging (IM), and Internet Relay Chat (IRC) to include some of the following features:
Whiteboard, Screen sharing, Application sharing, Video, Audio (VoIP), Chat, Session recording, Files/document library, support for smart phones and tablets.

 FreeConference.com (IBM) $7.50 / conference, $15/mo
 Web Conferencing | GoToMeeting® (Citrix) $49/mo
  zoom.us 40 min. free, $15/mo
 WebEx (Cisco) $49/mo

 Office 365 (Microsoft) was Live Office
    SharePoint Online – Collaboration Software
 Adobe Connect

IBM
  IBM Lotus Sametime
  IBM LotusLive Services
  IBM LotusLive and IBM Lotus Sametime Unyte Meetings Plug-in

Anymeeting.com free
Join.me free
Teamviewer.com free
Vyew free
Skype 
  Group Video Calling -  Skype 7 day trial
  Group Screen Sharing - Skype Premium $10/mo, $60/yr
  Collaboration With Skype Desktop Sharing -- Mobile Technology News
Dimdim free

Document Collaboration:
CheckVist $15/6 mos for pro version - outline format

Whiteboard Services - Interactive whiteboard software (IWB)
Dabbleboard - Online whiteboard for drawing & team collaboration - Interactive whiteboard software 
Scriblink - Your Online Whiteboard
skrbl: easy to share online whiteboard
Many others, See:
6 Free Online Interactive White Boards | Emerging Education Technology
drezac.com-  Ten Interactive White Board Web 2.0 Tools

Voice Only:
TeamSpeak - Welcome to TeamSpeak