Sharepoint - Is there a free version available in SharePoint 2016?

Unfortunately, there is no a frank alternative for SharePoint Foundation at SharePoint 2016.

But you can still use SharePoint 2016 as a trial for 180 days using the following trial keys based on your selected edition:

• Enterprise trial product key: NQGJR-63HC8-XCRQH-MYVCH-3J3QR
• Standard trial product key: RTNGH-MQRV6-M3BWQ-DB748-VH7DM

After the trial version has expired, You should be aware of

• You will not be able to extend it.
• You should provide a SharePoint product key via Central Administration. However, you still can add lists, items in lists, but you can't create pages, sites.....etc.for more details check EXTEND SHAREPOINT TRIAL PERIOD
• You can build a new environment with a new installation that I think it may be suitable for a development environment, but it will require additional work to prepare it and restore your farm/custom solution every 6 months.

I also listed all possible alternatives at SHAREPOINT FOUNDATION 2016 WHAT’S THE ALTERNATIVE?

As per following article

Previous releases of SharePoint Server included SharePoint Foundation, a free edition of SharePoint that included most of the core functionality and architecture provided by the commercial editions of SharePoint. SharePoint Foundation is no longer available in the SharePoint Server 2016 release.

What's deprecated or removed from SharePoint Server 2016

There is a trial version of Office 365 that is available for free.
You have 30 days of full functionality + 30 days grace period. The subscription is available here and allows you to create 25 users + unlimited (almost) number of site collections.

It is great for development and testing, but unsuitable for production unless you are planning to "jump" and move data from subscription to subscription every few months.