I really like applications that change the way we look at things.
Things like wikipedia which is an online encyclopedia, except for all articles are editable by users. So if you happen to be an expert on the history of your hometown, or the theory behind thermodynamics then you can enrich the listing for that so that other users benefit. Of course there are going to be shortcomings - plenty of people think they know something when in fact they don't - but that's the beauty of it: it is a self correcting problem. For every person who goes into posts and puts up incorrect information, you have somebody else going through posts fixing those items.
Outsourcing the gruntwork to the people who use the end product can be found in other instances as well. Take Google Image Labeler for example. You are paired with another person and then you are each shown identical images. You input keyword labels that come to your mind about the subject of that image. Once you and your partner insert the same label, you go onto the next image. This continues for two minutes. It sounds silly: but I have a lot of fun pairing up with somebody and seeing how many images we can get in that two minutes, and I have spent many hours doing Google's gruntwork of labeling images so that when you type "Batman" into google image search you actually get pictures of the black knight.
And since I love finding new music, there are a couple of great and intuitive tools for finding new music. The first one is called MUSICOVERY, and allows you to select the genres of music you like, the mood, and the tempo. So if you like dark, slow, techno songs, just make 4 mouse clicks and now are listening to songs that fit in that range. You are also looking at a screen full of other songs that fit into that same search.
Another one that I just found is Pandora. Type in an artist or a song that you know you like and after a couple minutes of the software doing it's thing it matches that song or artist with others like it. As each song plays you have the option to tell them if you like the new song, or dislike it. This not only updates their software with information, but also removes that type of music from your playlist.
Viva la revolucion!