Perfect

Brilliant little plugin that does exactly what is sais: it puts your site in maintenance mode and sends a “503 Service Unavailable” header, which tells search engines to come back later.

Nothing more, nothing less.

I prefer this plugin over other maintanance plugins as it doesn′t save any options to the database.

BTW: The previous two 1-star comments don′t make any sense. Look at the code, the plugin doesn′t do anything else than sending a 503 header.