Many of the web2.0 apps i've seen strike me as rather silly. Though there are some that are up and coming that i think have potential utility for our community. Here are a couple i can think of easily:

Wordpress allows anyone to set up a blog in a couple minutes, easy to use interface. Funny i'm typing this on a TypePad blog.

Ning looks promising too, a quick way to set up cool social and community apps.

I'll add more as i think of them...