The tool estimates using criteria from humans (del.icio.us tags, Alexa Rank, mentions at Wikipedia, etc.) and machine-assembled data (like pages indexed, internal link percent and even PageRank itself). It is by no means perfect or entirely accurate, but it is a significant upgrade from Google's often inaccurate and infrequently updated PageRank score (and it's fun to use).
Out of idle curiosity I fed in threadwatch.org and discovered the page strength as measured by the tool, and that Threadwatch
The sort of results are results like this for TW. It says the score is 7 out of 10 and
You're running with the big brands and sites, making content that engines and visitors can't help but gobble up. All that's left now is to leverage your power and push ever onwards, towards utter ubiquity.
Given TW readers love of gadgets, any views from readers on its usefulness ?