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.