I know xray vision is illegal on the server. What else is? For example I like the auto switch mod that changes your tool to the most efficient tool when u strike a block. I couldn't find a list of mod do's and don'ts Thankd
Anything that gives you an unfair advantage isn't allowed. (This includes hacking and texture packs - It isn't limited to just mods.) Probably easier to ask about a specific mod before use if you are really unsure. As for that tool-switch thing, I think others use it, and it hasn't become a problem..
Texture packs that are like "x-ray" are illegal. Other wise just normal texture packs that just make the game look better are fine.