I am really frustrated with Flickr's maping service, especially the new one they rolled out a few years ago. I wanted people to see at a quick overview everywhere that I have been and geotagged photos. You can't put all of the photos on the map at the beginning because it will be too crowded and slow.
MarkerClusterer does all the heavy lifting for the clustering (why I picked Google Maps). If you click on a marker or zoom in, it replaces the marker with several smaller markers. I had to suck down all of my geotagged photos to my database (about 11,000 photos) and I print a JSON version fairly quickly from my db.
Improvements I Made
I did a few improvements to speed it up. I made clicking on an item load the data by AJAX so the data is not always there. I also combined markers that were in the same exact position into "gangs" to cut the number of points from 11,000 to 4,000. This was a challenge because I had to get MarkerCluster to include the points and their counts.
Limitations and Going Forward
It initially only works for my photos. I plan on someday allowing other users to input their flickr photos.
Flickr employees - please take a hint and change that awful pubic mapping system.
You might have some issues in older (non-JS optimized) browsers since there is a lot of heavy lifting going on in your browser, especially if the person has many pictures taken in different locations. (Multiple pictures taken at the same exact location do not add to the performance cost.