Scrapbook
I find that when I am browsing my aggregator I am not in the mood to read lengthy articles. I'm just browsing/skipping through the new articles.I had a great problem that I would open up a lengthy article that I would want to read later, only to have to shutdown the laptop and loose all those wonderfully lengthy, and detailed articles lost to oblivion. The firefox extension above is a wonderful tool that allows me to capture pages, easily for later reading. Obviously I could have done a "file->save as", but I did not want to liter my hard drive with articles and, well, that was just another added step that got lost due to laziness. The extension adds an option to the right click menu of firefox, called "Capture Page". This saves the page somewhere (hopefully in the .mozilla or .firefox folder), and you can access it through the View->sidebar->Scrapbook (or Alt-K) for later reading. It just improves the blogging/browsing experience a lot.
Did I mention I'm addicted to firefox and all its extensions :)