oneforty seems to be "THE" app directory for Twitter related applications.

Quoted: oneforty - reviews and information on the best Twitter apps

All the Google Twitter Accounts listed here.

Run this script in Firebug to get the list:

a = document.getElementsByTagName('a');
r = /twitter\.com/i
for (i in a)
{
s = a[i].href;
if (r.test(s))
console.log(s);
}

Another interesting conversation widget - placed on a web site to display Tweets back to the same domain (and threaded into a conversation).

Quoted: TweetMeme finds the hottest stories from twitter for you to retweet

This is a cool list of the people that Robert Scoble has "favorited" one or more of their tweets (I made the list with 1 tweet...)

Quoted: On Favstar.fm - Who @Scobleizer has faved recently

The FourSquare application for Android is open source!