I was browsing my subscriptions on Google Groups today and noted an interesting module:
The little side module was tracking my signed-in activity, and the tracking didn’t amount to just Google Groups browsing. See that link to CNET Networks? That was the link to Google Finance, and indeed, a few days ago, I checked out the latest blog postings on CNET on Google’s Finance property.
That tracking is near and dear to me, since the same type of tracking is enabled on Yahoo! Tech site. Whenever you browse or search for a product, the information stays on your log, and both Google Groups and Yahoo! Tech allow you to clear it, if you don’t feel comfortable with that info being on the screen. 
If you use Yahoo! Local a lot to find reviews of good restaurants or dentists, you will notice a similar looking module on Local as well. The idea is that a few days later you might want to come back to the item in the database you visited.
Here’s however, the crucial difference – clearing the cookies (or alternatively, closing the browser and letting the cookie expire) erases the traces of your activity from Yahoo! sites. Even if you never bother to hit Clear, sooner or later you will close the browser, and on the next visit to Yahoo! get a totally different set of cookies, so there’s never really a decent log of what you’ve done and visited. It doesn’t seem to be the case with Google, which ties up the activity to the user id, and hence messing with the cookies would produce no visible effect on tracking.
Moreover, Yahoo! chooses the model, which separates tracking on per-site basis. Your recent activity on Local is never shared with Tech, and won’t appear on other Yahoo! sites, as far as I know. Google’s tracking is network-wide from the start, as my recent tracks in the system include read Google Groups and a page on Google Finance. Not sure which approach is best for the user, as in cases like this you trade convenience for privacy, but this is one of the interesting facts that’s just useful to know.
