Sam Bull

Add a mailto link to your site so that every visitor gets a unique disposable 
address, generated through the TrashMail service.

TrashMail Contact Me

Generate unique disposable contact addresses for each visitor.

moxu

I created this plugin because i wanted this feature (to set a specific locale for frontend) out of my themes. I’m assuming i’m not the only one who would preffer to have the backend en_US only, so here it is…
It’s not tested with other versions of wordpress except 3.4, but it should work without a problem at least from 3.0 onwards.
This plugin assumes you’re not using a translation plugin , so, if you have bilingual or multilimgual blogs, use with caution (and let me know … especially with qtranslate 🙂 ).
I’t not multisite tested/ready yet.

LanguageBack

Simple , minimalistic plugin that lets you set different locale for frontend and backend.

TrashMail Contact Me vs LanguageBack