This page has a few ISO, CDI, NRG, etc. images. There are going to be more eventually, but they will only be small things no greater than a couple MB a piece. Also, in .cdi format, you need DiskJuggler. In .bin format, CDRwin. And the others I don't have yet, so they aren't on here.
.cdi & .iso images are images of cd's that you can burn on a CD and play. When you put a CD in your computer, and you want to copy it, the program you used will make that "cd file" a certain format. e.g. DiskJuggler is .cdi, CDRwin is .bin/.cue, etc.
Also, you will need WinRAR to extract the files, as they are all in .rar format. There is a link on my main page to download WinRAR.
This is an image for CodeBreaker on the Dreamcast. You will need a memory unit to use it though.588kb
This is an image for Gameshark CDX v3.3 on the Dreamcast. You will need a memory unit to use it though.800kb