If you use App.net and Apple's Messages on OS X, you're going to want to take a look at Project Amy. The project is the brainchild of Steve Streza and was released at the App.net hackathon that took place last night. It allows users to integrate App.net private messaging with Messages on OS X.
MacStories has taken a look at Project Amy and delves a little deeper into how it works:
Project Amy is a clever experiment that showcases the App.net API and Apple's IMServicePlugin.framework: using only public APIs and no proxies (unlike Steve's first App.net project). Project Amy adds App.net as a native account in Messages, allowing you to have private conversations with other App.net users inside Messages. Once installed, you can simply login with your App.net credentials to have the account listed alongside AIM in the bottom-left corner of Messages; choose Window > Buddies (or hit CMD+1) to show a list of users that you can send private messages to (they will be displayed as online with their full names rather than usernames).
Interested users can download Project Amy here. Just keep in mind that at this time the project is still in alpha. Users should also not that Project Amy only works with Messages on OS X and not on iOS.