I just answered a question about this on Trulia and thought I would share my thoughts here on my own blog.
Craigslist remains a very popular forum for searching homes. The best part about craigslist is that ads only last 7 days so they tend to be more up-to date than Trulia or Zillow.
That said, the real estate section in San Diego has gone to the dogs in the past year and there are huge amounts of spam on it. It takes a great deal to wade through it and at this point, I would suggest using Realtor.com or the website of a Realtor. These are directly linked to the MLS and updated daily.
Loads of people search on craigslist and there is nothing in and of itself wrong with it. In past months however, it has turned into a free for all, and you honestly tell the difference between a real ad and a scam.
The very best source of real estate inventory is the MLS. Every agent’s website is a direct feed of MLS data. So, using an agent’s website is probably the best recommendation I can offer.