There's a saying that goes something like, "you'll never make any real money working for anyone but yourself." As true as that is, owning your own business can be a 24/7 undertaking, which may not be worth it to you. On the other hand, if you have aptitude for playing card games, that could be a fun way to make money.
Earn Money Playing Cards
When most people think of making money playing cards, they think of poker, either online or in a casino. Not many people think of earning money from home games, for a number of reasons. For one thing, the stakes are usually too low, and for another, the games are supposed to be friendly. Card "sharks" are usually frowned upon by the other players, and frequently they aren't included in the game if they continuously take the other, less proficient players' money. Still, if you're looking for easy pickings and don't need to make too much, these games can pay off.
Of course, to make real money, you'll need to play more frequently than just Friday and Saturday nights and for bigger stakes. Online poker is a convenient way to do this. There are a number of websites, such as fulltiltpoker.com and pokerstars.net, where you can play as many tournaments as you have time for. Just log on, establish an account, and join in.
The advantage to playing poker online is that you can do it in your own home. The disadvantage to it is that you're not at the table with your opponents, so if you're a good reader of people, you can't use that skill to help figure out what they have. To take advantage of the "tells" that other players sometimes give away, you'll need to go to the nearest poker parlor or casino. Many municipalities have legal poker parlors, having adopted the position that poker isn't gambling but rather a game of skill played for money. If the place you live doesn't share that enlightened view, you'll need to get to the nearest Indian reservation or possibly Las Vegas or Atlantic City. Many professional poker players have taken up residence in one of those cities for convenience.
If you've taken the time to get to a casino, blackjack is another option for you. Blackjack is a house game, meaning that the odds are stacked against you, but you can change the normal house advantage into odds that favor you using simple card-counting techniques, varying your bet according to whether or not the odds favor you. Be discreet about doing this, however, because pit bosses are continually looking for card counters, and if they catch you, they'll invite you to leave. You can also play blackjack online, and it's a good idea to do that for practice before actually going to the casino.
Finally, If you understand and play the game well enough, you can also make money from contract bridge. Large cities such as New York and Los Angeles have bridge clubs where players can play rubber bridge for stakes ranging from a dime to many dollars per point. But the easiest way to make money playing bridge is to establish a reputation as an expert at tournament bridge. Lesser players (known as "pro dates") seeking master points will pay you to play tournaments with them, knowing that your presence in the partnership will give them a better chance of finishing "in the money"--qualifying for master points. Some tournament bridge players have become quite well-to-do this way, and on the flip side, there are pro dates who have won world championships, carried there by their pros.