Check out Paint Tool SAI and FireAlpaca.

I always forget if SAI is free or not, but if it's not it probably has a trial program you can use just to see what it's like.

FireAlpaca is like a watered down version of SAI, and it's what I mainly use to do my drawings. I love it because of its Pen Correction feature (a feature shared with SAI) which smoothes the path of the brush as you're drawing. So if you have shaky hands that result in wobbly and uneven lines on your drawings, the Pen Correction tool is a really nice way to help correct that.

Yeah, SAI has a 31-day trial.  FireAlpaca seems really neat-but the only problem I see is that it's in Japanese.  Does it have an English feature? 

Japanese? Hmmm. The licensing stuff that pops up whenever you open the program is in Japanese, but everything else is in English for me. I downloaded it straight from the website and it runs in English for me.

Okee then!  I'll go ahead and try it out. 

Downloaded it!  Looks beautiful and easy to use, English-too!

I use Fire Alpaca. It was easy for me to learn and doesn't have any big bugs in it.