Wikipedia and YouTube are actually the most popular widgets that users add to the search sidebar when we do not include them by default. That surprised us too since those results are usually surfaced well be Google.

We think that grouping the results by source actually makes it easier to filter through the mass of information - you usually know where you are most likely to find the information you need, it just seems easier to go to Google to do the search.

Flickr and Twitter results are other popular sources to add.