If you have ever worked on an email campaign, you know that testing your flyer in all the different mail clients can be a time consuming and difficult task. Campaign Monitor has an all in one test which previews how your email will look in 17 of the most popular email clients, as well as testing it against some spam filters and firewall protection programs. Unfortunately it's not free. It's $10/test. Probably still worth it if your are working on high profile email campaigns. Way to see a need and fill it Campaign Monitor creators. (Found through Swissmiss).