You can check out the sizes I had made in
IslandDollars for campaigns. I tried to get the most common sizes, as well as some of the odd sizes. It depends if you just want them for sigs, or if you want them for ads too. There were a few sizes not common depending on the board. The bottom one here is 240x60. Another board was 728x90. The one on Ynot was a square of some sort.
I like Mighty Chin's suggestion of the PSDs, for those that would take the time to make them.