The best way to be in-the-know about current listings, and get updates to listing changes immediately, is to work with a qualified realtor. For people wanting browse multi-family listings, services like Loopnet are useful, although registration with the site is necessary. For the serious buyer, it's best to work with a realtor who has access to the MLS.
The Investors page of Dwelling San Diego has my contact information, the brokerage I work with, and Loopnet is embedded so you can get a quick glance of some of the current listings.