Typically mods are/were made available as freeware by the author (the copyright is retained, and you get access to the files for free). As long as you're not making profit (ads or otherwise) I doubt anyone minds really.
Take a look at
Nectarine, especially FAQ and Policy sections.
How well this would work for US website (or rather the US legal system) I wouldn't know...
Technically speaking you should not redistribute the mods unless if you can find the "license" for the particular file (basically some note from the author saying that the mod is freely distributable somehow, pd, freeware, CC or whatever, or alternatively get permission from the author directly). This, if you want to be anal about the legality...
