A neat little tool from meta search tool Dogpile that compares the top 10 results from Google, Yahoo and Ask Jeeves. The graphic shows how many results are in only 1, 2 or in all 3 search engines for a particular search and includes sponsored links from the top of the pages.
Dogpile have also produced a white paper (PDF format) on the subject entitled Missing Pieces: A Study of First Page Web Search Engine Results Overlap. The findings are no great surprise to many of us but it is always useful to have our experiences confirmed by experimental data. I find that the degree of overlap varies with the search: sometimes it can be as low as 2-3% in the first 100 results whilst on other occasions it can be as high as 90%. If you want to compare a wider range of search tools and their top 100 results try Thumbshots Ranking.