A huge list of Orkut/opensocial applications http://opensocialdirectory.org/wiki/Main_Page I saw a couple of other pages where you can get a list of applications. I will be updating this list when I get more urls.
If you want to know how you can add the applications to your orkut profile check out this post
These are some of the applications which I like.
1) .. will update later
Now for some ideas I have. A cool looking wall where friends can draw on the canvas. Add pictures and other things. It could turn out to be a complicated widget but it would work.
A widget to use the awesome new service drop.io. If you visit my drop [http://drop.io/vivekjishtu] you can drop files, links and notes on my page. And all of it without filling up a single sign up form.
A twitter widget where a user can post and keep in touch with other friends. I was making it but still not done with it. Anyhow the next application to be completed would be drop.io