Twitter has begun enabling a long-rumoured feature that will allow you to download every tweet you have ever sent. The archive is also searchable, and tracks retweets as well as your original messages.
Annoyingly, this isn't an option everyone will be able to use straight away; Twitter says the rollout will take place over the "coming weeks and months". (It hasn't appeared yet on my account.) To check if you can access it, click on the gear icon, choose Settings and scroll to the bottom of the screen to see if "Request your archive" is available.
Your Twitter archive [Twitter Blog]