I don't think swap magic can be considered a mod, it's simply an exploit disc, and the word exploit is used because it allow a use of the ps2 that is not intended by Sony, but it's not really an exploit in the sense that the software (Swap Magic) is used as it is intended to be used.
It can also be considered a software when using the .elf file in addition to free Mcboot
An example of an exploit on the ps2 would be 007: agent under fire, which consist of injecting codes that doesn't belong to the game by swapping discs, which is possible because the ps2 doesn't check the authenticity of the disc once the game is loaded.
An other example : the famous SQL injection http://xkcd.com/327/