iOS: Our Twitter feed is probably the closest thing many of us have to a daily journal, yet Twitter doesn't provide a way for us to easily export the data or search our own tweets. Tweet Keeper is an iOS app that will liberate your data from the service and make it completely searchable.
All you need to do is give the app a username or import it from your iOS Twitter accounts, and it will quickly go to work saving old tweets. Unfortunately, Twitter's API only allows it to access the account's last 3200 tweets, but that likely covers several years for most casual users.
Once the tweets are saved, you can view them all in a list, perform searches, and export them as plain text, a spreadsheet or a JSON archive. If you haven't yet crossed the 3200-tweet threshold, this means running the app occasionally will allow you to store a complete, searchable archive of your Twitter history. Given Twitter's recent policy changes, now might be a wise time to take control of your data.
Tweet Keeper is available for $1.99 from the App Store.