Top 25 websites from 2011 (and from 2010 and 2009) chosen by AASL at an ALA presentation in New Orleans. There are a number of collaborative tools.
All of these are free or partly free tools with some great applications. For example, there is something that looks like a cross between Prezi and MindMap called SpicyNodes and a citation tool called iCyte.
Check out the winners at