Klout attempts to measure influence both online and real using an algorithm. They display your Klout score and you can market that, they also offer “Perks” to influencers based on their scores and topics.
The Google Extension for Chrome can be a nice tool for looking up someones Klout score while on twitter. It can also be a good tool for returning K if they tweeted about it. Here is the link to get the extension: https://chrome.google.com/webstore/search/klout