The ticker symbol for NASDAQ 100 Unit Trust. It's like an index fund in that its price tracks the the NASDAQ 100 stock index with no human intervention (read, no moron overpaid mutual fund manager to make bad guesses). Unlike a fund, though, there is no minimum amount that you have to purchase, you can sell it short, buy it on margin, or trade options on it. In other words, it's just like a regular stock, but its performance is not tied to any one company, rather the market as a whole (therefore, one could argue that a 100% QQQ portfolio is more diversified than other one-stock portfolios).

You do need a strong stomach and a steady trigger finger, though. Like a couple of days after I bought my first batch of 'cubes' as QQQ's are sometimes called, their price took a semi-historic dive. There was a lot of handwaving in the media, then the correction was over, and the price resumed its insane rate of ascent. If I had panicked and sold, I would have been a sucker.