What's Your Mac Missing?

So after a bunch of people have asked me what apps they should get for their (usually new) macs and such, I decided I should probably start actually recording this information rather than just tell everyone verbally.  That way I don't have to keep telling everyone the same information and I can avoid forgetting awesome apps.

Therefore I'm gonna be starting a new line of posts with the label "Mac Must Have" signifying it's an awesome app you should probably set up for your application.  I'll also comment Mac related informational posts with this tag when relevant (such as a post on how to get your google voice number ringing on your mac).

So here's the important aspect:
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Coming up: Caffeine.