It turns out there is: It's called GIMP. That's an acronym for GNU Image Manipulation Program. It began in 1995 as a project at UC Berkeley, but it's come a long way since then!
In fact, it has many of the same capabilities as Photoshop. There are plenty of filters, brush tools, gradients and more. GIMP even lets you use layers like Photoshop.
In the past, GIMP was criticized for being difficult to learn, but the latest version has an easier interface. Even so, if you're used to Photoshop, GIMP may seem unfamiliar, but it isn't too hard to learn.
To download, just click the big orange download button in the header of the site. Then select the link underneath "GIMP for Windows."
Once you've installed GIMP, I recommend checking out the Help tab in the top right of the toolbar. Select "User Manual" to view seven helpful guides to the basic features. You can also read daily tips that can unlock very useful features in GIMP.