Government crackdowns have discouraged many potential gamblers in the United States from using online poker and sports betting Web sites. But an increasing number of people have found a legal way to play the odds online through daily fantasy Web sites.

These sites, which allow users to wager thousands of dollars on the performance of professional athletes each day, have been protected under a federal law exempting fantasy sports from restrictions on online gambling. They make up a booming industry that has taken hold among players and is beginning to attract the attention of investors. FanDuel, a popular daily fantasy site based in New York, recently raised $11 million from a group of investors including Comcast's venture capital arm.