Effect modes are easy to add. I've changed the halftone effect to dither 50% (and also added dither 25% and dither 75%) these apply the current colour / brush filtered through a dither pattern, so allowing transparant drawing (unlike the old halftone). Also added something I think will be very useful, single colour stencil. This will let you draw as normal, but won't overwrite colours that match the current pen 2 (right mouse button). This is useful for drawing in shapes when the surrounding area is a single colour, without worrying about going over the edge.
I have the ideas.xls file which contains a lot of Surt's ideas for effect and such, but some are quite complicated so I'm looking to implement simple and useful effects at the moment.
No doubt I'd do more work on some totally different area of the program next..

(These changes aren't uploaded yet).