I'd also like through my own answer into the mix - not as an android app but as a service you could potentially use if you are an androind/gmail user. http://voice.google.com is a great tool that lets you send and receive texts through the browser as well as sending you transcripts via texts to your phone when people leave voice messages.
You are given a phone number (of your choice) which you can keep forever which in case you change phone numbers, you can always be contacted through that number (but you can block people by phone number and it says the number has been disconnected).
The idea here is that if you give that google voice number to your friend and they send you a text to that number, you will receive it in your google voice inbox as well as your phone and it keeps conversations grouped like you would expect google to do.