YubNub is a command-line for the web. After setting it up on your browser, you simply type "gim porsche 911" to do a Google Image Search for pictures of Porsche 911 sports cars. Type "random 49" to return random numbers between 1 and 49, courtesy of random.org. And best of all, you can make a new command by giving YubNub an appropriate URL.
Jonathan Aquino writes on why he designed YubNub: On a practical note, I was tired of setting up the same Firefox keywords on each of the 5 computers that I use. By putting my keywords into YubNub, I can hit "am mark twain" for an Amazon search, or "gmap vancouver" for a Google Maps search, no matter which computer I'm on.
The beauty of YubNub is that anyone can help to extend it. If there is an existing web service with a submit form, they can add it pretty easily (like I did with the Amazon example above). But even more interesting is the adding of complex data-processing services (like validating an RSS feed, or converting webpages to audio using text-to-speech).
This will really come into play when I implement pipes (e.g. "google jon udell | to_rss | xargs text_to_speech"). Now that is going to rock! And I don't have to be the one to make these commands -- anyone in the world can create the code for to_rss, xargs, and text_to_speech, hosting it on their server. YubNub is just the glue that enables these pieces to interact.