SRD75

Once the Memory Usage Monitor plugin is activated, activating or deactivating any other plugin will display the memory used by that plugin. e.g:
Memory usage for WP Memory: 0.02 MB
For accurate memory usage breakdown for each plugin, you must activate or deactivate them individually; you will not see memory usage when bulk activating plugins.

Memory Usage Monitor

Displays memory/RAM usage for each plugin as they are activated and deactivated.

Mike Jolley (a11n)

Testing your WordPress site? This plugin allows you to display memory, page load time and query count in the footer of your site on both the front-end and within admin.
This plugin is partly based upon the WP Query Counter by Steven Word (http://www.stevenword.com/plugins/wp-query-counter/) and has been customised for my testing needs.
If you’d like to contribute to the plugin you can find it on GitHub: <a href="https://github.com/mikejolley/WP-Page-Load-Stats" rel="nofollow ugc">https://github.com/mikejolley/WP-Page-Load-Stats</a>

WP Page Load Stats

Not sure why this only has 100+ users but this is a great tool!

Excellent!

Display memory, page load time, average load time and query count in the footer of your site.

Memory Usage Monitor vs WP Page Load Stats