Do you want to follow someone's updates on Twitter, but don't necessarily want them to see that you're following them? Or maybe you're reluctant to join Twitter, but still would like to see what someone is tweeting. You can actually add someone's Twitter feed to your RSS reader.
Just go to the page of the person you want to invisibly follow, and you'll see the RSS feed symbol underneath the people they're following (alternatively, if you're using Firefox, you can just click the blue RSS symbol in the nav bar). Just click that and you'll add their updates to your reader, just like any other blog's posts!
Invisibility cloak achieved. You can do the same thing with personal blogs, Flickr accounts, or any web page that offers an RSS feed.