To use Soulver, type in a math question as you would write it out on paper. Soulver provides the answer as you're typing, ranging from simple math and currency conversions to complex problems. The awesome thing about Soulver is that you also can type in words and Soulver knows what you're talking about. What to know how many miles are in 100 meters? Type "100 meters in miles" and you'll find it's roughly .06 of a mile.
Soulver can keep track of stocks as well, but you have to program the app first. Once your preferences are set, tell Soulver the stock symbol and it spits out the current stock price. Or, if you want to purchase ten shares of Apple stocks and you're in the UK, type in 10 AAPL in Pounds and it tells you that it's currently £2,539.02. It's not a sophisticated scientific calculator, but can do basic algebraic equations, functions and other bases.
Soulver is $24.95 and is available via the Acqualia site or through the Mac App Store. iPad and iPhone versions of Soulver also are available.