A very small tax on all financial transactions will have little effect on most people, but will make high frequency trading and many kinds of speculation less appealing, which will reduce dangerous gambling in the stock market.
Here's a summary of a 2008 paper by Dean Baker:
"The recent economic turmoil has generated renewed interest in a financial transactions tax (FTT). While such a tax will be vigorously opposed by the financial industry, it offers a very attractive mechanism for raising revenue that is arguably efficiency-enhancing. Calculations based on 2000 trading volumes showed that a set of scaled transactions taxes, imposed on transfers of stock and other financial assets, could raise more than $100 billion a year, even assuming large reductions in trading volume."