Between 2010 and 2012 or so I picked up some condos here in San Diego at short sale for about 1/3 of what their price was a few years earlier. I get about 1.5% of their purchase price every month in rent. At the same time, the property values have appreciated so the rents are starting to increase as well.
The longest I've had any of them vacant was about two weeks and that was only during the time I was replacing carpet, appliances, furnace, painting walls, fixing stuff, etc.
To make it completely passive I have a property manager (I live in the area, but I value my time). That along with HOA fees and real estate taxes eat into my bottom line, but combined it's only about 1/5 of the monthly rent.
These properties allowed me to quit my job, self-fund my company, and I'm actually putting money away every month. Go figure.
I'm not a real estate expert, but if you have any basic questions feel free to get in touch (contact info is in my profile). As background, I bought my first house at 21 and owned 5 homes by the time I was 27 (I'm 29 now). I was in the military until a few months ago, so I didn't make a whole lot, but I'm pretty good with money and invested wisely. I didn't grow up with much, so I learned what not to do with money. I'm also pretty deliberate about how I spend my money, which is different than being frugal.