One suggestion: Check out what feedblitz.com offers. You provide a feed (or in your case, feeds by category). FeedBlitz gives you a little form to add to your theme that folks can enter in their email and sign up to receive emails of your content when updated.
You could also use the WP Email Notification Plugin. It allows you to choose which posts you send to subscribers, and you can keep your email list in-house rather than with an outside service like Feedblitz. Nothing wrong with Feedblitz. I’ve used it myself but enjoy having my email list in-house.
The 3 best offsite solutions are Zookoda, FeedBlitz and FeedBurner. Each offer subscription services for members. Zookoda’s is the most powerful as a list, feedblitz has some nicer stats, and feedburner is great if you’re already using it for other things (ie: why use yet another service?).
