Avocado slicer
From Cookipedia
An avocado slicer is a simple tool which makes light work of cutting and preparing avocados. You cut the avocado in half and remove the stone - there is no need to peel it. You place the slicer over each half and draw it through the flesh towards you and you have perfect slices. Chop or mash the slices for use in guacamole recipes.
Avocado slicers are available from most good cookware shops and from numerous online outlets. The one illustrated is made by Kitchen Craft.