What software/program does hotpads use???

I need to create a program/software like hotpads, where my users can search a map and see where little tabs on the map for rental apartments etc... I would allow my users to create their listings that will then show up on the map and be seen publicly.

How would I go about doing this? Is there a software i can buy or would i have to build it from scratch because I don't think I have the programming skill to build something like that alone.

anyone? I will give mad REP!!!