Twitter (2006) is AIM reinvented (1997).

Joost (2006) is Broadcast.com reinvented (1998).

Facebook (2004) is Friendster reinvented (2002).

Justin.tv/blogtv.com/every 5th startup (2007) is web-cams reinvented (1996).

Seems nobody has reinvented blogging yet ... hmm...

Steve shared with me a fascinating statistic:

Are wikis gonna go mainstream soon?

Update: Check out the comparison when you add forum to the list. If there's 3 things that every company needs, it's a forum, a wiki, and a blog.