If you don't have a Google Voice number yet, you're really missing out. I managed to get my number back when it was still a "by invitation" beta product, and it's now open to the public. Google Voice offers more features than I can go into here. One nice example is the ability to have it ring multiple phones (home, cell, work) when you get a call and instantly transfer the call to whichever one you pick up on. In terms of the Android app, you can listen to your messages without having to sign into your voice mail. Google Voice will also transcribe your messages (often times with some hilarious mistakes). It's not a perfect transcription, but I've found that I can almost always get the point of the message, which is very helpful when you're somewhere that it would not be socially acceptable to put your phone to your ear to listen to a voicemail.

COST: Free except when using for international calls