I see where Google Voice is mentioned several times, but I don't think it has been emphasized as a must have application. First, Voice is a big step, but worthwhile. The big step is that you are issued a new number for life. You don't absolutely have to have a new Google number, but many features will not be available if you don't get a new Google number.
Sure, it's a pain initially. You have to tell everyone that your number has changed, etc., but you need to treat this as a new number because you have to train your friends and family to call you on this new number. The new Google Voice number aggregates all your phones into one number. Thus, your home number, your work number, your cell number are all used with Voice. If you get a call to your Google Voice number, all your phones ring simultaneously. No forwarding, no multiple voicemail boxes to check.
When you get a Voicemail on your Google Voice, you get a transcript of the message that was left in your Voice mailbox. Also, you can get an email transcript to your email. You can have multiple announcements on your voicemail depending upon who is calling: One for family, one for friends, one for business or everyone else. You can even have an announcement for your girlfriend and another one for your spouse.
Voice is integrated into your Android phone. You can elect to disable your regular text program (suggested) and use only your Voice program for texts. Thus, your voicemail and texts are combined into one list.
There are too many features to list, but here are a few that are cool: When you receive a call on your cell phone and you get home or to work while you are on the call, you simple press * and the call will be transferred to your landline. (The caller doesn't notice.) You can add an extension to your Chrome browser that duplicates the Voice features so you can make long distance calls free (you dial the number in the browser, it calls your landline and when you answer it connects the long distance call). You can also text from your browser and it appears as if you are texting from your phone.
Here are all the features:
Features - Google Voice Help
Hope this helps!