I was getting frustrated that there wasn't anything as good as Preev for STEEM, so I built one myself.
It allows you to enter your amount in any currency, and instantly have it converted to a different currency; e.g. STEEM Dollars to USD, and vice versa. It's very fast and easy to use. Unlike other conversion tools out there, this is a very simple page, no crazy moving backgrounds, and no endless amounts of Javascript.
value.steem.network
Just type in your number at the top, and select the currency pairs, and it'll update instantly. Zero waiting.