Salesforce.com's Marco Casalaina wrote a cool new Chatter-based application recently called ChatMetrix that I wanted to highlight for developers interested in developing with Chatter.
This visualization application allows you to browse the recent chats you, your followers, and those you follow have made by keyword search. In the upper left hand view you get a tagging field showing you the most popular terms in the last seven days. This tag field is clickable, updating the graph in the main view in the lower half of the screen, showing who in your network has used that tag recently and how they are related.
The upper right pane shows the contents of the Chatter posts which contained the currently selected keyword term such that you can scroll through and see what is being said. Additionally, in the upper left the user can tab over to a bar chart showing with decreasing frequency the most commonly chatted keywords. Again, the bar is clickable and will udpate the relationship graph in the lower half of the interface.
This sexy interface is great to play with, and could prove to be the basis for quite a lot of follow on applications. Some ideas that come to mind for me include a trend finder, expertise locator, hot topics indicator, to name a few. You can pull down this application yourself from the AppExchange here. Install them on your own app and see for yourself the kind of great interfaces you can integrate with Chatter.