MyTwitterButler is a minimal cost application, $10 per installation, that will auto-follow people based on keywords and also send mass direct messages. They are also currently being sued, more on that later.
This is a .NET application, so yes it is for you Windows users. It is sent with the license around the main Twitter username you register, but they allow you to add more names once you install. Unlimited apparently.
So this will sit idly by making API calls and auto-following people based on hots from the public stream. It can also be used to send direct messages to everyone you follow in common in bulk. Good for large notifications. However, bad for spammers trying to send directs to everyone.
I viewed the FAQ pages, which were well done, and the UI is simple and straight to the point. A la ‘not fancy’. The site owner spent a good amount of time trying to answer anything you might think of from reading it.
Apparently the site owner is being sued as of Aug 11, 2009 for utilizing the API to bulk follow, which breaks some rule in the Terms of Service. Also for using Twitter in the site name itself. We will be watching the result of this.