-clarifications
By digital painting I mean paint the traditional way on a computer, excluding the way you would use MS Paint, that is you have pencils, watercolors, albeit all digital version.
–Photoshop
So far my own experience with Photoshop is limited to amature level photo retouching following some random tutorials step by step, but I’ve already felt the unlimited freedom and how both beginners and masters can enjoy it.
Previously the main use of PS is photo editing, but apparently Adobe tried to extend this territory. From recent version (Since CS5 or earlier?) PS added a brand new digital painting perspective, together with the assortments of brushes (i.e., draw media and wet media) to simulate traditional media, PS is becoming a more and more suitable and pleasant environment for digital painting.
-Tip: if you want to learn digtial painting, STOP trying different fancy brushes! Stick to one (like chalk) and learn all thing define the word “painting”: perspetive, value, composition, through that single brush. Trying different brushes at the begining is same as steping at the same location – you feel you are learning but you aren’t!
I haven’t started using PS yet, but for anyone want to take digital drawing/painting seriously, I think PS is a must-have. And please see some videos of masters’ using it before you give it up (for its price).
-Tip: classic ways are always crucial, although there are no undo-redo, no layer mode, but these ‘deficiencies’ simply can never undermine its value. It is the way has been followed by great artists all over the world, all through the time. Besides, this is no distraction from modern electronic device, believe me, it feels really ‘quiet’ when you are holding a pencil and draw traditionally.
-Tip: you can’t have your laptop and tablet everywhere with you, but it’s possible with a pencil and sketchpad, I think the first step towards an artist is to start carry your drawing tools with you when you start a trip.
–Corel Painter
Simply, it is not suitable for my needs. The reason is I have mypaint and Photoshop already. I once tried Painter but felt it’s user-unfriendly, and the softwere is slow on my machine. So I don’t think I’m going to use it, though the brushset is more complete than any other software and many artist like it.
–MyPaint (http://mypaint.intilinux.com/)

MyPaint screenshot (work by David)
Free and powerful, that’s my impression of this software. Mypaint is a free, open source software that I with to recommand to everyone. Everything was designed to be as simple as possible (GUI is the simplest of all I’ve seen, also there are many single-key shortcuts calling the most-used functions like eraser, canvas flip, color pick, etc.), so when you work, there is no need to look for any functionality (whick is distractiong, by the way) – they are right under your left hand (assuming that you are right-handded and your right hand is holding your digital stylus.
The are also many choices of papers and brushes. Personally I like David’s brushset a lot, which is embeded in mypaint by default, along with other well made brushes.
-Tip: as to paper, I would suggest you NOT to stay on the default white paper for ever. Try something else, like the yellow-tinted paper which some professional designers use to show their production level work. Sometimes, a change of paper do give you a ‘pro’ feeling :-/
Another thing worth mentioning is this is a fast evolving software. On youtube and its offical website, the number of artworks created with mypaint is steadly increasing. The development is very active too (you can check their git commit log if you wish to look into the code), and new funtions are being added. The next big step to take for the developers seems to be layermode, some of which are already implemented. I havn’t chekc that out because currently I only use pencil, but for people working with colors, you may want to download the source code and compile the newest version.
Simplicity does not mean function limited, you can create professional drawings with this software.
–One Great Resource
Please Google Fengzhu if you don’t know about him, you can find his blog and the school of design he tutors. Also search him in YouTube for about his 46 free video tutorials (http://www.youtube.com/user/FZDSCHOOL).
Fengzhu is not only great in his art skills, but also in that he shares all his experiences, including how he use certain tools and the real-time working process. There are ~40 videos in his YouTube channel and new upload generally comes every two or three weeks. I can assure that you can learn a lot.
-Tip: all videos are accompanied by vocal explanations, so many tips are ‘hiden’ in a random minute of a random video. For me, as a beginner, I would watch them all so nothing would be missed.