Shally and I have long advocated that you want to check multiple search engines for any given query, because the results can vary dramatically, even for the exact same search string. Therefore, you'd be missing out on great results if you only depended on one. In 2005, I attended a web conference where I discovered Thumbshots Rankings, which allow you to graphically see the low overlap between search engines which proves this -- and we've been using it in webinars ever since!

But now I have to thank LinkedIn megaconnector Vincent Wright for turning me onto Blind Search, a spare-time creation of Microsoftie Michael Kordahi. This tool displays the first 10 results of your desired query on the major search engines (Google, Yahoo and Bing), each in their own column, but doesn't identify which engine provided which set of results! You need to vote for the column that you think provided the best results, and then it shows you the source for all the columns.

The results are often surprising. I ran a simple yet narrow resume search (but you can enter complex strings) and found that Yahoo was better than the others. If you're curious, you can see the first 8 weeks of cumulative results, but if you think that's like looking ahead for the answer key to a test, you may be disappointed:  The aggregated results can't make conclusions for any single person, but if you use the tool, you may prove to yourself that one engine gives you better results for certain things (e.g., resumes vs. contacts, depth in one industry vs. another).

One could argue that Zuula has given this ability to compare results between search engines for years, but: 1) Zuula makes you look on separate tabs to see the results of each engine; 2) Zuula doesn't blind the results first, so your subconscious bias can influence your opinion of the results as you click a given named search engine's tab.

In any case, this shouldn't replace the habit of sourcing against multiple engines, but if you're in a pinch and have limited time or for some reason can only search in one place, the Blind Search tool may cause you to change where you go first!