Well there are different kinds, like pear or cherry or whatever. Name a fruit and someone has made a cider out of it. But they are still (most likely) apple based.....cause you know...that's what cider IS.
Not too necro and old post, but...
The term cider is specifically for fermented beverages made out of apples. A "cider" made of pears is called perry. Pretty sure a fermented drink made out of cherries would just be a cherry-wine (or sparkling cherry-wine if it has bubbles).
Not that terms like "pear cider" haven't been used by professionals to market their product, but it isn't a correct use of the term cider. Some countries have laws that say in order to label a beverage a cider a certain percent of the contents must be apple juice and there are trade organizations that have similar policies for their members and/or labeling acceptance.