The first public beta of both iOS 10 and MacOS Sierra is now available to download, for free, without having to sign up to any developer programs.
That means that you save a huge $99 a year, however, you have to remember, that like the developer releases, it is a beta. That means that there is some risk to downloading either of the betas.
But, if you cannot wait to try out iOS 10 or MacOS Sierra, then you can try it out right now before its release this fall.
There are some differences between the public and developer versions, the developer version has been designed to include all the features needed to develop, whilst the public version is more about Apple getting feedback.
You should take note that these public betas should not be installed on your primary device, especially if you wish to continue using it without bugs.
Before you go, also make sure to make a full backup of your current OS.
You can find out more about both betas and how to sign up to them at the source link below.