You may be familiar with App.net, a service designed to be an alternative to Twitter. Originally a subscription-only service, App.net now has free tiers on an invitation-only basis.
App.net has an open API which allows developers to do interesting things. One of these is Project Amy, by Steve Streza which allows you to use Messages in OS X to send and receive App.net private messages. If you use App.net, you can download Project Amy here.
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