the one that I have, that I think is the best, is free and called "SMS Backup"
What it does, is, it basically forwards your SMS/MMS messages to your gmail account. The genius of it is that it archives them and labels them "SMS". So they skip the inbox, thus avoiding a "new mail" notification. It shouldn't even be that hard to set up a filter in gmail so they're automatically marked as "read" too, although skipping the inbox is good enough for me. It threads them just like gmail does with regular emails. The subject is "SMS with _contact name_". I haven't seen what it does with MMS, but I imagine it is the same.
While this does not allow you to restore them back to being text messages, it is way better than Txtract (the free one doesn't work with cupcake, thanks dev!) which converts them to a file type you can't easily open on your phone. You can have this run in the background (which looks like it can take a couple minutes to do on it's own) or you can do it manually with the app.
I really do love this app, because now I can just delete threads and texts all the time without really having to worry too much about losing them. One of the things that can make your phone laggy is having lots and lots of text messages saved on it. So clearing out your SMS and your browser cache/history can keep your phone running smoother.