Orite has created an online tool to emulate HTML/CSS rendering capacity of various Email clients. It references the Email CSS support Guide from Campaign Monitor, and utilizing the email-standards.org Email ACID test page as the example. You may paste in your own email template to test in.
Please note, this tool only provides a brief demonstration, by no means to be the exact reflection of how email clients show emails. Please remember this tool highly relies on the browsers capability as well, so be sure to use a standard compliant browsers. For more solid tests please try Litmus Email Testing or the Campaign Monitor Email Testing tool. If you want to convert any external CSS to inline style, Inline Styler and Emogrifier may come to help. Here goes the FREE tool, post your comments.
Currently tested under FF3/3.5, Safari4, Chrome2/3, IE7/8, Opera9.6