The only reason people can't install client mods is because they are scared to do anything technical. If you don't know how to do it just look at some tutorials or just ask one of us who know how to do it.
I can give a simple tutorial here for how I do it on my mac.
In terminal make a temporary folder in your user directory
Navigate to the new folder
Extract the minecraft jar to that folder (you can just type "jar xf" and then drag the jar into the window
Code:
jar xf /Users/your_user_name/Library/Application\ Support/minecraft/bin/minecraft.jar
In finder add the necessary files and DELETE THE META-INF folder.
Then update the minecraft jar with the new files.
Code:
jar uf /Users/your_user_name/Library/Application\ Support/minecraft/bin/minecraft.jar [b]./[/b]
OR
The lazy way is just to just download something like
MCPatcher and have it do all that for you.