Google Calculator & units and currency converter

http://www.googleguide.com/calculator.html

<Super>C - toggle dialog
<Escape> - close
<Up>/<Down> - navigation
<Tab> - activate suggestion
Left-click or <Enter> - copy result to the clipboard
<Delete> - remove result from the history

You can use '$' to convert currency with default parameters (can be changed in the settings).
For example with default USD to EUR:
42$ - convert 42 USD to EUR
42$$ - convert 42 EUR to USD

Screenshot