I've had trouble finding free, ad-supported apps that didn't suck. Do you know of any that are of high quality?
TapDefense, especially since the ads stop showing up if you get a bunch of "favor" which is where you download/run free games to get new maps and towers overall its a pretty fun TD game with a bunch of different modes, but the ads DON'T show up during the main game which is the most important thing IMO. It shows up at the end of the level at the bottom.
The only add supported games I keep on my iPod are Badaboo and VIP Valet. Do old NGMOCO games count? There's Topple (1&2) and Dropship which are pretty good.