This website is a small part of a project written for 24hbc by Erik Olsson.
The project was to finalize the engine behind this, not to build another map website.
The clustering takes place directly in the SQL query, you may see the actual querytime in the response using firebug or similar.. (between 0.005-0.04s)
The design is not very good, but on the other hand, the performance is very good ;)
Big thanks to Blocket for allowing me to use their data. Also available with data from Booli
Update: Please compare this solution to the commercial solution from Maptimize.