This option is not exactly free, however I thought it was worth mentioning.
Year ago you had to pay fees for using Tradestation, like $100 per month or something like this, now there's no charge. Although you must open an account (I don't recall the account minimums).
You do have to pay a small monthly fee for certain data feeds (i.e. Futures), although can you can always import your own 3rd party data. They have some of the most mature backtesting software, including Walk-forward optimization.
If you do use their data network, you have access to 1-minute bars, seconds, ticks. You can also run standard parameters optimizations, and you can run it inside of Windows VMs if your a Mac user. You can also scale it across multiple machines in offline mode, which is powerful if you want to run optimizations in parallel.
To code your strategies, you must use EasyLanguage. There's also the Optimizations API, where one option allows you to provide a XML file with the optimization parameters that you want to run.
For years I resisted Tradestation for various reasons, but I finally caved and now have learned to love it. Its very powerful if you know how to use it.