I’m thinking about embedding a banner (no popups) on my webmail system, does anyone know of a well-paying banner program that allows me to pick categories of banners?

Try www.adbility.com for that sort of information. It's not the same as when ad guru Mark Welch was running it but it's still the best bet for finding a suitable ad program.

You may also want to use a program like Commission Junction (www.cj.com) and take the categories of ads you want from their lists of categories, then put all the ad codes into a database and have your own code rotate them throughout the webmail.