If you are trying to add your own jQuery code to WordPress, and have had the error “$ is not a function” show up on Firebug, here is the fix:

Convert all dollar signs ($) to ‘jQuery’
The dollar sign is reserved in WordPress for the Prototype library, which is why it errors out. For example, instead of:

$().ready(function() {
$(“#select_me”).show();
});

Change it to:

jQuery().ready(function() {
jQuery(“#select_me”).show();
});