The first hour of the trading day is called amateur hour. Overnight Market orders have stacked up, and stocks are most volatile during that first hour. Avoid trading during amateur hour. The last hour of the day, often referred to as the power hour, is when the pros trade. Daily dips often occur right after amateur hour, but seasoned traders wait for the stock to begin moving up again before buying.

 
The site Money.cnn.com agrees:

If you watch the first hour of trading, it's like the Wild West, "said Bill Burnham, a general partner with venture capital company Softbank Capital Partners. Along with the close, it's the most active part of the trading day. Most investors would do better to wait until the market quiets down before placing a trade, he thinks. "There's clearly less hanky-panky going on after the first hour. Source: Skipping trades at the open  (or here as a PDF document doc.1437).

The Investopedia website (or here as PDF doc.1438) also discusses the best time to trade in the article What is the best time of the day to trade? This article also recommends, because of the greater volatility, that less experienced investors avoid trading at the beginning and at the end of the trading day.

First thing in the morning, market volumes and prices can go wild. The opening hours represent the window in the Market which factors in all of the news releases since the previous closing bell, Which Contributes to price volatility. A skilled trader may be able to recognize the appropriate patterns and make a quick profit, but a less skilled trader could suffer serious losses as a result of this volatility. If you are a skilled trader, then you may want to consider trading immediately following the opening bell, but if you are less skilled as a trader, you may want to avoid volatile trading during these hours.
The middle of the day tend to be the most calm and most stable period of trading days. This is the time of day when people are waiting for further news to be announced. Because most of the day's news releases have already been factored into stock prices, many are watching to see where the market may be heading for the remainder of the day. Because prices are relatively stable during this period, it's a good time for a beginner to place trades, as it may assist in more predictable returns.
In the last hours of the trading day, volatility and volume increase again. During this part of the day, traders may be trying to close some of their trades, or they may be attempting to join a late-day rally in the hope that the momentum will carry forward into the next trading day.
The time of day in which a trader enters into a position can affect the outcome of the trade. Inexperienced traders or Those Who Want to steer clear of the price volatility of trading early or late in the day should consider placing their trades in the middle of the trading day.

See also the website TradingSimulatorSoftware.com (or here as a PDF document doc.1439); the  BracketTrading.com site (or here as a PDF document doc.1440);  and the site BetterTrades.com (or here as a PDF document doc.1441) .

We also call this indicator the amateur / professional indicator.