How often do you let your subscribers know of changes to your website? For many people the answer is never. Whether it is because they never got to it or they do not have the capabilities to do it, being able to update customers of changes to your website could be a great business opportunity.
When I do consulting for other companies I recommend them to update their customers about 3-4 times per year of anything new. Some companies send out eNewsletters about once per month which is good. It is not hard to have a programmer build a program that sends out constant updates. There are also programs in real estate specific such as Top Producer or in general like SilverPop that can schedule email champaigns in advance which is something good to look into.
Tonight I was reading about a service called ChangeDetection (ChangeDetection.com), which notifies your customers of changes to your website. It seems to be something basic. At least it is better then nothing and it is free.