In no particular order:

- DSi Klub. I'm biased on this one cuz I made it. :P

- DSi Plaza. Had the classic minimal, IP-based version as well as the later versions with extra features. Both were good for different reasons.

- DSi Cade. The one that got me into web development, even if I barely used the site later on.

- DSi Paint. Cool stuff with painting and later sprite creation, never did much with the community though.