I recently discovered another neat feature in Google which I believe has been there for years but somehow managed to remain hidden from me for a long time.
Google with do conversions for you in the search box. Try typing any of these phrases:
15 gallons in liters
(also accepts the non American spelling of litres)
4 hectares in acres
3 nymphets in strippers
(Okay the last one doesn't work yet. I guess that Google hasn't finished adding all the conversion tables.)