Almost all the betting sites I know are offering a free spin, it's become a normal popular thing already, cause any of them are likely to offer it.
Most of them really offer a free spin.
If not free, they give you a free dust through faucets and let you bet that dust, just for fun. If you lose you lost dust, and if you won, you just won a useless dust...
Yolodice for example allows you to do that.