I've long known that Google will convert between units if you type a query into the search engine (or your browser search bar), such as "50 miles kilometres". I hadn't realised that if you simply use an equals sign, Google will make an educated guess at the unit you're aiming for.
As Google Operating System points out, this works well if the query is relatively unambiguous: search for "50F=" and you'll be given the temperature in Celsius. The reverse doesn't work so well, since "30C=" draws on C as the speed of light.
The default assumption for currency conversions is into US dollars, so if you search for "100AUD=", you'll find out how much your money is worth. Search for "100$=" and it will convert from US dollars into Google's best guess at your local currency (it picked Australia correctly for me). All up, definitely a useful timesaver.
Quick Unit Conversion in Google Search [Google Operating System]