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.

Sergey Biryukov

Creates a separate tiny .mo file to use on a site front-end. Allows to save some amount of RAM on a shared hosting server.
Thanks to MAX for the original non-plugin solution and to AlexPTS for the idea.

WPLANG Lite

Creates a separate tiny .mo file to use on a site front-end.

Memory Usage Monitor vs WPLANG Lite