TweeterID
TweeterID allows you to easily look up any username (@handle) on Twitter and find out what their corresponding Twitter ID is. This indicates the order in which the account joined Twitter.
Although often used as a tool to compare the order in which certain people joined Twitter, this tool has become incredibly useful for programmers and developers working with the Twitter API to quickly check and determine the Twitter ID and associated Twitter Handle of certain accounts while testing and developing apps that integrate with the Twitter API.