A clever hack that puts old Google and new Google results side by side, so you can use both: CompareGoogle. Useful? Not really. But kind of entertaining if you're a search geek.
I prefer, though, the blind taste test of search, BlindSearch, created by Microsoft employee Michael Kordahi. You enter your query and it gives you three panes of search results, from Google, Bing, and Yahoo, but in a random order and with nothing telling you which is which. You find out what's what once you tell it which result set you like best. How scientific. I ran it several times and found Google to be my preferred engine, although Bing was close behind. (Thanks, SFGate.)
Other side-by-side search mashups include YahoogleSearch (Google vs. Yahoo); Twingine (also Google vs. Yahoo); Bing-vs-Google (guess); Blackdog's Google-Bing (Google vs. Bing in multiple languages); and BingTweets (Bing plus Twitter Search);
There are also several services that grab results from multiple engines in the background and present them in one collated list. Examples include Dogpile, which I used to use when Google didn't give me what I wanted, and Clusty. See also our overview of metasearch engines.