I understand a lot of you use Paint Tool Sai like I did, and I thought I'd share two free alternatives in case your looking for new features or if you need it on Mac or android.
I've been using krita for the past couple weeks. Krita has a great deal of fan freaking tactic layer tools and realistic brushes. However it more closely resembles photoshop than Sai IMO (at least the older versions do). It has more of a learning curve, and installing custom community brushes is a must. If you are comfortable only with Sai I still suggest checking it out, but it might be a little too different for most people. Krita is available on PC Mac and Linux for free.
medibangpaint is perfect for the Paint tool Sai enthusiast. It is available for virtually every OS (including android and Ipad). I'm pretty sure they used firealpacha's code base, somehow. The interface is slick and very reminiscent of Sai. Medibangpaint adds more tools such as materials, text, and gradients while retaining most everything from Sai. TBH i haven't played around with it too much, but I'd definitely suggest checking it out. Paint tool Sai IS aging, and it doesn't look like they'll be supporting it much in the coming years.
Do you have any free art tool recommendations? What kind of tools are you comfortable using? What about equipment?