For those of us with unsteady hands, or used to graphing/grid paper for drawing, this feature would be most appreciated. When trying to make a series of 16x16 blocks, it's rather annoying to have to constantly make SURE I'm not one pixel off, or have the mouse move JUST enough when I click to offset my work. I caught a mistake in my project far too late thanks to this, and as a result, have had to scrap the entire map. An entire day's work, gone. Not to sound like an angry ranter: but for those of us who want precision in our map editing, a grid system and ability to snap the cursor to the grid would be a GODSEND.
A grid display would, preferably, be customizable, IE: 2x2 grid, 2x3 grid, 16x16 grid, etc, with the ability to snap the drawing tools like the brush to the grid. (Preferably with snap options that would allow for 1 block of spacing as well. IE, so you can put multiple 16x16 blocks together, but each block is still 16x16, instead of say, connecting two 16x16s and ending up with a 31x16)
Another nice set of tools would be those you find in most painting apps: lines, shapes, etc. Trying to draw a straight line with a pencil tool on a mouse is near impossible unless you have the world's steadiest hand, so a way to draw them easily would be wonderful.