In "Mail" (which is not an iMac specific app, btw), compose a new email. In the new email window, you should see "Stationary" in the upper right corner. Click this and you will see options for many stationary templates to use. I think this is what you are referring to. My wife loves this feature.
Additionally there may be some confusion with the old Mac.com (pre-mobile me). The old mac.com had many different greeting cards from birthdays to holidays and everything in between. I used to love sending birthday cards, Christmas cards, get well cards, etc. When Apple changed to Mobile me they did not offer that feature. That was the feature I loved the most. Thanks a lot Apple! The stationary feature in Mail is very nice and you can easily add photos from your iPhoto library but it is limited as far as greeting cards are concerned.