This is an client side alternative to the Facebook Multi-Friend Selector that 
relies on jQuery. No server side component necessary which makes it really
easy to adapt and use. 

Check out this blog post with more details and screencat: http://bit.ly/cHDkzm

To see a demo go here:
http://mbrevoort.github.com/jquery-facebook-multi-friend-selector/#
Click login and login to your facebook account and you should see the friend selector.

Much can be customized by CSS, for example if you want a stacked list type selector that's more stripped down:
http://mbrevoort.github.com/jquery-facebook-multi-friend-selector/list.html

This plugin is depends on the new Facebook Javascript API
<script src="http://connect.facebook.net/en_US/all.js"></script>

There's a jquery plugin and accompanying CSS file

Assuming you have a container like:
<div id="jfmfs-container"></div>

And you have included the Facebook Javascript API
And you have already logged the user in. (FB.init and FB.login)

You can load the friend selector in a container like this:
$("#jfmfs-container").jfmfs();

This should fetch the current users friends and give you the interface to select friends. Then when you're ready to move on, there's a function to call that returns an array of the Facebook Ids of the selected friends. 

var friendSelector  = $("#jfmfs-container").data('jfmfs');
var selectedFriends = friendSelector.getSelectedIds();

Options 
These options can be passed into the jfmfs function with a map of options like jfmfs({key1: val, key2: val})

max_selected: int (optional)- max number of items that can be selected
max_selected_message: String (optional, but required if max_selected specified) - message to display showing how many items are already selected like: "{0} of {1} chosen"}