I use the free sketchbook program that comes with the modbook, but I've also purchased ArtRage and the Adobe Design Premium package.
I also use a bunch of free programs such as:
Blender for 3d work—the 2.54 beta is really nice.
Pixen for pixel art (I'm involved in a few indie game development projects)
Also, I'm finding Alchemy ( http://al.chemy.org/ ) to be a very valuable tool in all my projects. It's a vector based program which enables me to produce some really quick concept sketches. There are also loads of different effects that you can fiddle about with in order to create some interesting shapes/patterns etc.
You could at a push just use this program to produce some really effective artwork, but as it doesn't have an undo option, that could prove to be quite a bit of a headache—although, I seem to be using it more frequently and for longer periods of time.
I particularly like using the pen pressure option and the mirrored effect, it's tripled my concept art output that's for sure.
You can export as either bitmap or in vector format for further editing elsewhere, which is really handy if you like working with vector programs such as Illustrator or the free program Inkscape ( http://inkscape.org/ ).
I really can't big this one up enough.
Another great free program that I've come across, and that I've just started to get to grips with is Sculptris ( http://www.sculptris.com/ ). It's currently a windows based program, so you could use bootcamp to access it.
Although, here's a link ( http://www.zbrushcentral.com/showthread.php?t=91628&highlight=mac ) to a working mac port—I say working, it is missing a few minor features such as texture mapping (which you could do in another program such as blender if you like).
Basically, this is a really neat sculpting app that again compliments the modbook greatly. Go check it out.