I'm always intrigued by backup tools for Web services that don't really need backup. TweetSaver is no exception. This paid service backs up (almost) everything you've ever posted to Twitter, along with private messages and replies from other users. It then adds an extra layer of utility on top of it, like a search tool that's limited to just your messages, simple sharing options for each message, as well as a way to assign a tag to each tweet for categorization.
Of course the usefulness of all of this hinges on Twitter being down and/or somehow losing all of your data. However, based on Twitter's uptime over the past year, you have to ask yourself if it's worth the cost of $20 a year. That's pretty steep considering there are some free Twitter backup solutions like TweeTake and TweetBackup that do many of the same things.
It's also worth pointing out why you wouldn't need this service for some of the features it's advertising. For one, Twitter's search tool can be limited to just your tweets if you use the from: operator. TweetSaver is also only capable of retrieving your last 3,200 tweets, so if you've got more than that, they're not going to be archived. This in itself is a limitation of Twitter's API, but should be noted if you have 40,000 tweets and think it, or any other service, is going to be able to grab them all.
I do like the idea of tagging tweets though. Crafty Twitter users have already found subtle ways to tag their Tweets by using hashes (#'s), although there's no way to go back to old messages and add them in. It can also be impossible if you're running out of space in an outgoing tweet. Below is a demo of how it works. I'm getting access later tonight and will update if it's got any hidden goodies that make it worth the price: