I believe free online slots are the best games to have fun on slots. After all, you don't need to spend any money to play free slots, do you?
Some websites take this free fun to the next level by letting you win money as well. Such websites award you free money once you register on them. This free money is called the sign up bonus. Once you get it, use this free money to play slot machine games.
The fact that you get to play real slot games for free, nobody can stop you from winning. So, pick a slot game, insert some coins in it, feel lucky, and click the “spin” button.
Once you win, your winnings are added to your game account from where you can withdraw or carry forward them, in case you decide to stick to the same website.
One thing though, most genuine websites let you withdraw or carry forward a certain portion of your winnings. (After all, they won't like to go bankrupt!)
Still, play free slots if you want to have slots fun and win money for free. Enjoy!