An online collaborative genealogy tree that would connect all the persons who live or have lived. With photos, biography, maps… The best trial is probably werelate.org.
A Google Earth based collaborative historical atlas with links to sources, illustrations and videos. Should include various maps like frontiers, demography, battles… Something like that :
A platform for online collaborative science that would allow everyone to propose a project or to join one. Should include wiki, Tex, peer-review system, bug report, photo gallery, etc. I already discussed this here. Colabscience.com could be a good starting point.