Tons of stuff in XBL mate
I had one legit and one account just to mess with. In L4D and Gears especially I had some nasty mods patched in the .iso such as infinite ammo, no reload, auto firing shotguns, spawn menu for literally everything, god mode, stuff like that. In GTA IV there was "heavy indestructible cars" option that caused sooo many lulz in MP
I "acquired" all picture packs from the store without paying, tons of XBLA arcade games, things like that. By that time MS were using very poor hardware ID protection method so you were able to change your console ID to blank, making it unbanable. This was during the time when some guys figured out the 48 hours xblive cards algorithm and they released the generator so I used that too for short while, but MS fixed it rather fast. Fun times indeed, it was lovely to mess around with the kids online
I don't think such things are possible anymore though, the current anomaly detection systems are way more advanced and probably you will get caught in a hour or less. For altering the .iso that is.