Photography gels are precisely colored acetate sheets (overhead transparencies) used to alter the color of lights. There are different reasons for doing this, including for aesthetic effects and contrast reasons. Professional sets can cost a fortune, these cost me $14.25 (not including the transparencies, which I already had), and are great for any amateur photographer!
Step 1: Making Them
I went online and found the standard color values for a set of gels. Using these, I made an 8x10in jpg file for each color. Attached is a zipped file containing all 27 images that you can burn onto a CD.
Once this is done, take the CD and a pack of acetate sheets to Kinkos, or a similar printing venue. Kinkos only charged me $0.49 a print.
Easy as that.