Yahoo Instant Search is a feature launched by Yahoo last year, in September. Instant Search uses Ajax to deliver instant answers to your queries, so it's optimized for searches that usually trigger Google's Onebox, like: unit conversions, math calculations, weather, local time or simple questions.
Yahoo's Instant Search is not that smart as Google Calculator, but it's definitely worth trying if you don't like to press the Search button over and over.
Some queries to try:
* time in Paris
* weather in Boston
* convert 100 pounds to yen
* convert 5 kilograms to pounds
Errata: the title should have been "The Fastest Online Calculator and Converter".