Don't fool yourself into thinking this is something that one person can do with a moderate work ethic and a month of free time. A team of developers and a large amount of knowledge about server-side and client-side technologies are required to build and maintain that map....at least one that is that high traffic and with a database as complex as MLS.ca
If you have a handful or very defined items that you want to plot on a google map, however. It's pretty simple, all you need to do is look at the Google Maps API
For the record, MLS.ca uses a Bing Map, not a google map.