I don't think Paint Tool Sai is free. Opencanvas 1.1 is still available for free download, however. It's a similar sort of program to Sai quite small.
Many art programs have decent trials, as well. Photoshop has a thirty day trial, as does Illustrator and Painter, if I recall. There's a nifty program called Art Rage that has a free trial, too.
There's GIMP, for a 100% free alternative to Photoshop, but I'm not a fan of it (probably because I've used Photoshop for so long).