This is a hideous tool that can be used to export your entire Twitter timeline to a CSV file, readable by any spreadsheet application (Excel, Numbers, etc.). Exported data includes the tweet plus a timestamp of when it was sent.
In less than two years it has created the underlying structure for more than 20,000 social-networking sites--"mini-Facebooks" with an aggregate of 300 million page views per month. You've probably never heard of it.