Krita is a painting and image editing application for Windows, Mac and Linux that can be a very good competitor for commercial apps like Photoshop. The best of all, Krita is free, although I would advise you to donate to the developers if you think the app deserves your support.
When I first launched Krita and created a new document, the first thing I did was what I always do: try the different brushes and check for stylus pressure support for my Wacom 4. I am happy to inform that this application fully supports this feature, as you can see in the image above.
The application also has a lot of different brush presets to choose from. You can then change the settings of the brush by using the toolbar controls, to fit your needs. You can change things like color, size, opacity, or blending mode.
Another interesting feature, also available in Photoshop, is the ability to create shapes using paths. After you draw these, you can edit the points to change the shape of the path, and it gets updated in real time.
Of course, Krita would need to support images, because there are situations where you will need to use pre-made images for various reasons. Importing an image into Krita is as easy as dragging onto the canvas. The image is added as a new layer and you can also change the blending mode of this new layer. As a note, you can change the blending mode of any layer.
Another feature some would use a lot (me included) is the ability to add transparency masks to layers. Fortunately, Krita fully supports this. However, by the time of writing this, I still haven't figured out if the transparency mask can use a copied image (meaning, copying an image to the clipboard and pasting it as a mask instead of a layer), something I do a lot in Photoshop.
Krita also offers a wide variety of filters that can be applied to layers or images. Filters include color adjustments (brightness, contrast, saturation, levels, HSV), blurs (regular blur, gaussian, lens, and motion blur), artistic (like oil painting, posterize), edge detection, emboss, and others. One thing to notice is this application doesn't include as many filters as Photoshop, so it may feel limiting in this aspect.
If you were to switch from Photoshop to Krita, the first thing you'd need to know is whether Krita can open your Photoshop files, and it can. It even supports some layer styles, which is a good thing.
Krita can also save your projects as Photoshop files, which is pretty much a must, since most applications will support Photoshop files, but not many (if any) will support Krita files. For example, Unity supports Photoshop files just fine.
Now, the big question is "should you switch to Krita?" If you are on a position where you can switch from Photoshop (or any other paid painting application) to an alternative, you should definitely take Krita for a spin and see if it fits your needs.
After all, the app is free so you have nothing to lose.
Get Krita now: http://krita.org/