Great! Rated 5 out of 5 stars
On content heavy sites, a lot of markers get clustered together. It's still kind of manageable as the markers are a bit transparent so I can see behind it. However, it would be even better if the markers actually move to the "top layer" as you mouse over them so that the labeling becomes readable.
P.S. Just thought of something. Adding a feature to have the markers only appear when hovering near the scrollbar and disappear when not would eliminate the need for a button that takes the markers away completely.