Geni.com is a site that I live but rarely used because it didn't allow GEDCOM imports. Geni is a social networking themed genealogy site. GEDCOM files are the standard genealogy tree file. GEDCOM allows you to share your tree with others and move trees from program to program. When Geni.com started, I couldn't believe that they didn't allow GEDCOM uploads. They finally allow it, but there's one caveat: if you already have an account with Geni, you have to start a new account. They don't merge the two accounts and there is no way to upload the GEDCOM to your current account (probably because they didn't program it to ignor ancestors already in the tree and merge information).
Anyway, Geni.com is now worth checking out again and I'll have to do another review. Right off the bat, I'm having difficulty seeing if anyone on Geni is linked to me.