Android: If you're rocking an Android phone without the Google Voice experience, you can still back up and read your SMS messages online. The free SMS Backup app automatically transfers your SMS messages to a Gmail label as they come in. You'll need to enable IMAP in your Gmail settings and give SMS Backup your Gmail username and password. After that, though, the app can back up the SMS messages you've stored on your phone to a Gmail label—"SMS" by default, but you can change it to anything you want. You can choose whether to have those messages appear as unread when they land in your label or not.
As Lifehacker reader Jeff wrote in his recommendation, it's a great secondary database to have handy. Ask new contacts to text you with their contact info, and you can grab it later from Gmail and add the details to your contacts right there. If you use a service with SMS reminders, it's a good place to keep track of them. In general, it's also a fun way to track the messages you traded back and forth with somebody during big events or frantic nights out.
SMS Backup is a free download for Android phones only. Be sure to grab the right app if you go searching in the Market—this app was written by Christoph Studer, and has a small Gmail icon in its logo. Use the QR code at AndroLib with your phone's barcode scanner if you want to be sure.
android-sms [Google Code]
http://code.google.com/p/android-sms/
