Matrix Collapse Plugin, put this in a folder called "matrixcollapse" in craft/plugins/

MatrixCollapsePlugin.php

<?php

namespace Craft;

/**
 * Matrix Collapse Plugin.
 */
class MatrixCollapsePlugin extends BasePlugin
{
    /**
     * Get plugin name.
     *
     * @return string
     */
    public function getName()
    {
        return Craft::t('Matrix Collapse');
    }

    /**
     * Get plugin version.
     *
     * @return string
     */
    public function getVersion()
    {
        return '1.0.0';
    }

    /**
     * Get plugin developer.
     *
     * @return string
     */
    public function getDeveloper()
    {
        return 'Bob Olde Hampsink';
    }

    /**
     * Get plugin developer url.
     *
     * @return string
     */
    public function getDeveloperUrl()
    {
        return 'http://github.com/boboldehampsink';
    }

    /**
     * Include JS to collapse Matrix Blocks
     */
    public function init()
    {
        if(craft()->request->isCpRequest()) {
            craft()->templates->includeJs("$('.matrixblock').each(function(){$(this).addClass('collapsed');Craft.MatrixInput.rememberCollapsedBlockId($(this).data('id'));});");   
        }
    }
}