Salesforce Chatter provides a foundation of collaboration features that you can use in your own applications that you build on Force.com.  The new Chatter Workbook, just announced here at Cloudforce 2010, shows you how to do this. It's a gentle introduction that walks you through four tutorials:

• Tutorial 1 provides a general tour of Chatter functionality – familiarizing you with the basic concepts such as status updates, users, following people and following data, and groups.
• Tutorial 2 shows how to create a native Force.com application that programmatically updates your status and performs a query on your news feed.
• Tutorial 3 shows how to create a Java application that interfaces with Chatter using the Force.com Web Sevices API – it performs the same task as Tutorial 2, but does it from a simple Java console.
• Tutorial 4 does what Tutorial 3 does, but uses .NET instead.

Tutorials 3 and 4 are also interested if you'd like a basic introduction to integrating with Force.com from Java/.NET.

Once you're up and running creating your own application, and integrating Chatter into it – you'll want to step it up.  In particular, you'll want to investigate the data model that underpins Chatter – and learn how to write queries that return the data you want.  

For that, look no further than the Chatter Cheat Sheet.  It's dense, but it's packed chock-a-block full of ready-to-use SOQL queries.  For example, it has queries that return your list of followers, or that return the data that you see in your news feed. We have laminated color versions here at Cloudforce, but the above link also points to a black and white version that's a little easier to print yourself.

Enjoy!