If I put a mod like, smart moving on my craftbukkit.jar, will everyone have to have it on their clients to go on the server? or will they just not be able to see/do it...?
they will need it on their clients to see it, otherwise it will look like you're moving around standing up
exactly. if you wanted, you could install zombe's mod pack on your client and use it on your server. Toomanyitems works as well.