I've been working with a lot of developers around Chatter apps.  People are doing a lot of neat things with the tool.  Speaking broadly, and keeping in mind that we're only a couple of weeks into the developer preview, what I've seen so far falls into eight distinct categories. 

Note: Most of these links will require access to the Chatter Dev Zone.  If you haven't already signed up, take a second to do so and experience it for yourself.  Also, many of them are new, so be sure to log in an check them out.

1. Chatter in brings data into a Chatter feed.  The data can come from anywhere, internal or external processes, but it always winds up in an object feed.  All of the ideas around Twitter -> Chatter import fall into this category.

2. Chatter out takes data from a Chatter feed and sends it to some other consumer.  The iPhone client Chuck G posted about is a Chatter Out app.  Side note: the iPhone client is awesome.

3. Follow more apps help users find more people or objects to follow.  These might include managers or executives, but it might also include a specific set of records or type of activity.  Steve Andersen's "Chatter Follow Rules" (below, find it on his profile) is a great way to do this.

4. More follows apps are a way of recruiting more users as personal followers or to follow records.  (Coming soon.)

5. Visualization tools help you see what's going on in the Chatter-verse.  They help you understand the social graph around a person or a record and what's happening in it.  Chatter Bubbles is a visualization tool for the general stream. 

6. There are a variety of analysis tools designed to show metrics about activity.  CDZ's Chatter Directory is a great example of this.

7. Discovery apps are a great way to see what people outside of your social graph are up to.  Chatter Tags on the CDZ is a good example of this as is Chatter Search.

8. Perhaps most interesting are the stand alone apps that uses Chatter as a message bus are also starting to emerge. Joel D's Task Manager is a great example of this.

These eight general categories describe most of the apps I've seen.  One of the aspects of this which I find fascinating is the diversity.  We're only two weeks in to the preview and we've seen a great deal of activity.  What will the Force.com ecosystem do when Chatter is released?

What kind of apps have you seen?  Better yet: what kinds of apps have you created?