This is not on the Top 10 list, only because it receives my "Best Geography Twitter App of the Year Award." This is a great site that combines the power of geography, social media and geospatial technologies. You can map where your twitter followers are (if they make the data publicly available). Additionally, you can pan, zoom and identify specific followers and map their networks. Can you find yourself on this map (followers of @APHumanGeog)?
Via Seth Dixon