There's a few threads in this part of the forum that deal with this; the most recent one I know of is here (
click for link to thread). To be fair it's been a few weeks since this thread was last posted on, but my post on the linked thread is still relevant and the free spins are still available on the below sites AFAIK.
FortuneJack offers 25 free spins just for registration from what I can see on the homepage of the site, and
BitStarz also offers 180 free spins on registration as well. If you're just interested in playing and/or testing out the games on a casino, some casinos allow you to play via play money or tokens like
FortuneJack or
Crypto-Games. Good luck!