In Meme tracking with Greasemonkey, Jon Udell introduces a userscript which grabs the number of references from del.icio.us and bloglines and appends that information to the top of every page you visit. To be fair on Jon, the version he has released defaults to only doing this for pages on Infoworld.com but modifying it to run on every web page is trivial.
The obvious downside of this kind of script is the amount of additional web traffic it induces. Every page you load in your browser induces an extra HTTP request to both del.icio.us and bloglines. Times that by several hundred users and those sites are going to be serving thousands of requests every minute.
Can this kind of thing scale? The Google toolbar retrieves the PageRank for every page you view, and the Alexa toolbar (and thousands of spyware applications) request information for every page viewed as well. The difference is that the developers host their own servers, and are responsible for their own bandwidth bills.
There are also very serious privacy implications involved in this kind of activity. Right now, Joshua Schachter’s del.icio.us access logs are collecting a detailed history of Jon Udell’s browsing history—and that of anyone else using the script.
This is a frustrating quandry, because the technique used in Jon’s script can be extended in almost limitless ways. Sadly, in a world where bandwidth and server resources are limited such scripts must be approached with caution.