The Mail program organizes stationery into categories, but it also includes a Favorites category where you can drag your favorite stationery so you won't have to hunt through the stationery categories every time. If you find the stationery selections provided by Mail too limiting, you can buy additional stationery from Equinux (http://www.equinux.com/). If you have technical expertise, you can even create your own stationery by reading Apple's stationery creation guide, "Creating Mail Stationery Bundles" (http://developer.apple.com/documentation/AppleApplications/Conceptual/MailArticles).
If you want to spice up your email messages with signatures, create several signatures that display a saying or phrase to share with others. By creating several signature files and letting Mail choose them randomly or in sequential order, everyone receiving your messages can receive different signatures.
If you're adding a signature for business use, include your company name, motto, and website address to promote your business every time you send email. Since recipients may forward your email messages to others (including the text of your signature file), this can be a free form of advertising.
Whether you use stationery or signatures (or both), there's no reason why your email messages need to be plain and boring. Now all you have to do is make sure your messages are worth reading in the first place.