That depends on what you mean with “for free”.
Play for free without the possibility of winning real money or bonus money
If you want to play for free using play-money, there is a multitude of online casinos that will allow you to play an unlimited number of games for free, for as long as you want to. Most of them won’t even require that you register first.
There are casinos that will make you jump through hoops before you can try out their games with play money. e.g. by requiring you to complete a lengthy registration process that includes confirming your email address and registering a credit card. There is also online casinos that will limit your ability to play for free with play money, e.g. allow you 1 hour free play only or give you a certain amount of free play money that can not be refilled.
Regardless of what type of player unfriendly policy you encounter when you are looking to play casino games for free, remember that there is really no reason to patron such casinos, since there are so many better alternatives available today. There is a huge selection of excellent online casinos out there that will welcome you with open arms and not put any obstacles between you and the casino games you wish to try out for free.
Play for free with the possibility of winning real money or bonus money
If you want to play for free and still have a chance of winning real money or bonus money, your options are more limited – but they do exist. One way is to use no deposit welcome gifts. There are a lot of online casinos that will give you free spins or bonus money as soon as you complete your registration, without requiring a deposit. You should be aware however that many of these casinos will require a deposit before you can withdraw any money form your casino account.