I currently use Spotlight, but it's super non-ideal. I'll often dismiss spotlight and then realize I forgot the result. (I really hate what they did to Spotlight in Yosemite—it's overloaded and thus worse at its original purpose of searching the filesystem.)

I also recently bought Soulver (https://www.acqualia.com/soulver), which is great but not quite as quick as a dashboard widget.

Thanks for the modified upload—I realized the version on Apple's site has a visual glitch with the search bar length. I also found this version, which makes you hint enter before you can see the result, but feels more polished. Can't tell if there's any relation. https://www.apple.com/downloads/dashboard/calculate_convert/...