When Google Desktop first came out, it was moderately cool. You could search Outlook Mail, your files, Internet Explorer web history, and AOL IM chat history. I use Windows at work, so I tested it out. It worked from what I could tell, but I use Thunderbird for mail, Firefox for web, and Gaim for AIM chat. I totally forgot about Google Desktop.
Then Google Desktop came out of beta. Not only did they add Firefox and Thunderbird support, they supplied a plugin interface. This is huge. That enables developers to add Google searching to any product. For example, someone could write a plugin to search Gaim logs. It’s tempting to get a free t-shirt, but as I don’t have a Windows box at home, I doubt I will even attempt making a plugin.
That’s just a small case. Imagine the possibilities! Search your iTunes library. Search Opera’s web history. If there was a Windows equivalent of Delicious Library, you could search that. They’ve even made it easier to access it by allowing you to embed it in your taskbar or make it a floating window. Very cool. Very extensible. These plugins take a mediocre product and make it wicked cool.