Visual Voicemail puts the messages on your screen in a list. You can then listen to messages in any order without having to "call" your VM.
Youmail uses a internet site to redirect your VM messages to them. You then retreive them in a similiar fashion. YM also has a option to have your messages "transcribed" into text. Several other options as well.
I'm sorry, never even looked @ your provider. VVM for $2.99/mo is through VZW. Not sure what the cost will be through sprint/nextel. Most likely will be the same or +/- $1.00.