Show last modified date for plugins in plugin list.

t5-plugins-last-mod.php

<?php
/**
 * Plugin Name: T5 Plugin Modification Info
 * Description: Show last modified date for plugins in plugin list.
 */
add_filter( 'plugin_row_meta', 't5_plugins_last_mod', 100, 2 );

/**
 * Add en entry to the plugin description.
 *
 * @wp-hook plugin_row_meta
 * @param   array $meta
 * @param   string $file
 * @return  array
 */
function t5_plugins_last_mod( Array $meta, $file ) {

	$path   = WP_PLUGIN_DIR . '/' . $file;
	$format = get_option( 'date_format' ) . ' ' . get_option( 'time_format' );

	if ( is_readable( $path ) )
		$meta[] = "Last modified: " . date( $format, filemtime( $path ) );
	else
		$meta[] = "Could not read file time.";

	return $meta;
}