Ah, NO program has a blur quite like SAI's.
When doing the Mac thing, I've used Painter 11, Photoshop CS5, and Artrage Studio 3.
For Painter 11, I customize an Acrylics brush (use a grainy soft cover on a single stroke brush with 0% resaturation), although some prefer the built-in tool of BLENDER-JUST ADD WATER.
For Photoshop CS5, the 'smudge' brush can serve (but turn on 'scatter' with a low % for it). If you're feeling confident, you can also use the new blender brushes, which are excellent.
For ArtRage, you are pretty much limited to using a palette knife; great for some effects but hardly patch on SAI's.
You can also check out the 'smudge' brush on pixelmator, but I've never used it so I cannot tell you how it goes with that. Same with GIMP ...