The program I use is a free download called paint.net. It is the greatest thing I have EVER used besides a pencil and paper.
The only problem with paint.net is that it cant save as a monochrome bitmap. That is where my good 'ol MS paint comes in.
The paint.net is very similar to photoshop where you can use layers and stuff but also has some of the nicer features of MS paint. But there are tons of similar programs out there to choose from. I just found one that had the tools I need most and was free.