If you're a serious follower of the stock market — or if you just dig Gordon Gekko's suspenders — you might be interested in the NASDAQ Portfolio Manager, an official iPhone app release from National Association of Securities Dealers Automated Quotations (a.k.a. NASDAQ), the American stock exchange.
In its basest form, the app allows you to look up various stocks and get access to real-time data when the market is open. You can pull up a stock's chart of highs and lows, then rotate your iPhone to expand the chart and get additional viewing options. Run a finger over the graph to get stock prices at various points in time.
What sets the Portfolio Manager apart is its link to StockTwits, the Twitter stock service that returns tweets about a particular stock (using the @StockTwits $STOCKNAME syntax), as well as drill down into Twitter proper to find out what others are saying about the stock in question.
The only thing the app WON'T let you do is actually buy and sell stocks, but for the price of the app (free!), you're still getting a ton of market data that you can use.
[Download from iTunes]