I also now use Google for this sort of conversion, especially when I'm
away from my computer! A feature I'm finding more and more useful is
the ability to "Google" via SMS Text Messaging from my phone. It's fast
and when I'm careful in my search term selection, it often gives me the
info I'm after on the first try.
To use it, send a Text Message from your phone to "GOOGL" (46645) with
your search terms. In a few seconds you'll receive a test message back
with an answer.
I just tried it with the "10 F to C" example and got back a single
response: "Google Calculator: 20 degrees Fahrenheit = -6.66666667
degrees Celsius"