1. Google Voice
Google Voice is a service that is so useful I consider it one of the top benefits of Android itself. The service gives you a phone number that can ring to multiple places or devices and it allows you to access all of your voicemail and text messages from the Web. The Android app integrates even deeper. It can make outgoing calls look like they're coming from your Google Voice number so that you can keep your real mobile number private.
If you'd prefer to view this as a list instead of a photo gallery, then see the companion blog post.