adding a button to the media uploader

When i needed to do something similar i used the admin_print_scripts-media-upload-popup hook
to add my own js/Jquery code to insert a button and handled it’s click event with ajax.

something like this:

add_action('admin_print_scripts-media-upload-popup','add_my_media_button');
function add_my_media_button(){
?>
<script>
jQuery(document).ready(function() {
    //loop over all attachments
    jQuery(".savesend").each(function(i,v){
        if (jQuery(this).next().attr('id') != ""){
            //get attachment id
            var att_id = jQuery(this).next().attr('id');
            att_id = att_id.replace("send[", "");
            att_id = att_id.replace("]", "");
            //insert button
            jQuery(this).append('<a href="#" class="button add_to_album" rel="'+ att_id +'" id="add_to_album['+att_id+']" name="add_to_album['+att_id+']">add to album</a>');
        }
    });

    //add handler for click
    jQuery(".add_to_album").live("click", function(){
        //do  your thing here
        alert("add to album click");
    )};
)};
</script>
<?php
}