I don't own any fancy graphics design tools. I've tried a few and they always seem hard to learn and have many more features than I need. I am pretty happy with what I can do with Excel, MS Photo Editor and Picasa. I start with Excel using just borders, drawing tools and colors and a macro that makes square cells.
You can change the Excel color palette, which is key. Here is a simple quilt design I did in Excel and then pasted into MS Photo Editor so it is a .jpg file. It shows the colors, design and fabric requirements. I like the colors a lot but am having some trouble finding the right fabrics.
Sometimes I lay out a single block and take a photo of the block. It looks like this after it has been cropped:
Then I open several copies in MS Photo Editor and put them together. I can also rotate the blocks (the second row is made of rotated blocks):
I can get a pretty good idea of what the whole quilt will look like by putting lots of blocks together.
I also sometimes download fabric photos from on-line quilt shops/fabric manufacturers to use in this process. That helps me decide if I want to buy a fabric for my quilt. This doesn't work perfectly since the photos sometimes are not accurate representations of the colors, but it's still pretty helpful.
I use Excel for lots of design problems. I designed our kitchen cabinets with Excel. You can look at that here.