We bought in when the housing market was at its low. The credit markets were crap so we had to buy all of them cash except the one we live in now which was a bit more expensive. After expenses (HOA and property management) our average returns are about $900/mo. ($10,800/yr.) for every $100,000 invested. That's not too bad considering housing prices and rental rates will continue to climb over time (or that's the plan at least). We're fortunate enough to live frugally and just happened to have the money lying around when the housing market sucked so we bought a couple homes a year over the past few years.
For the house in South Carolina we're losing about $200/mo., but it's a 15yr mortgage. We'd be making a bit if it were a 30yr. I bought that one when I saw 21 so I didn't know anything about anything. Some would say I still don't.
The real estate is nice because the extent of my involvement is manually paying real estate taxes on the homes without mortgages, answering a couple emails a year about high dollar ($200+) repairs or signing new lease agreements.
As one of the other posters mentioned, we also invest in Lending Club. Our investment was more just play money to see how things worked, but returns so far have been surprisingly good, around 14% annually. I like it because it's so diversified with only $25/loan. You can maximize returns by picking better loans.
The passive income is nice because it'll allow me to quit my job next year and work on my startup full-time. I want to apply to Y Combinator and even if I don't get accepted I'd be able to bootstrap my startup indefinitely with our passive income. Hopefully it doesn't come to that, but if it does maybe I'll pivot around year 25 (year 50 at the latest).