...somebody pointed me to a program called
Deep Paint 2.0 the other day.
It seems to be a pretty good Photoshop clone.
The downside is that the company who produced it seemed to have stopped any development in it some years back.
The upside is that it's free!
I haven't tested it out but I installed it and had a look through and it looks pretty good, so if you haven't got PS or the budget to buy it, then this might be worth a try.