I've heard about it before but I always found trials only. I found one some months ago when I was making some "comissions", and some other pictures (that I still have not finished, as usual) I tried it and I loved it!
Here's a link in case you didn't know about it.

It's small, smooth, does not consume ridiculous amounts of memory and has some nice tools.
I had not enough chance to test it because when I remembered (this month) that I had it while trying to "ink" some pictures, I found that my trial version expired.

Ah yes, other than trying to write again, I am drawing again! Mostly because I got a special request from my friend Kiara, and I still owe to nekokain and Hinoki the picture of our RO Guild when we used to play togheter.

Anyway... We all know that I have nothing to buy stuff online, so, I went instead to search for a keygen or something.

I know we all have at least an illegal program in our computers anyway, that's why now I am sharing the link where I found a cracked and full working SAI tool I found today (and in english! ^^).

By the way, just in case, I did NOT upload it and neither taking credits for it. I am just sharing the link

Well, here's the link to Easy Paint Tool SAI from Taringa!, uploaded by TeamAquaDan, for anyone who reads this and would like to give a try to it if have not done it yet.
I highly recommend it, it's nice.

Enjoy.