Affiliate marketing if you do you keyword research correctly.
I think it's important that most people realize that with passive income, there is no such thing as 100% passive. There's always initial work or maintenance involved.
Passive income is more so creating something of value that requires minimal to no effort by the recipient to maintain.
Aside from some aspects of real estate, I'd have to say affiliate marketing can be the easiest and best passive income sources around if you're willing to put in the initial work. It's also a very low barrier to entry on it, so anyone can do it without a lot of costs.
If any of you guys are interested, there is a really good passive income IRC chat community to hangout and discuss/collaborate about it.
My retirement stock account invested in large index funds. I guess my savings account earns 1% interest also.
Any other source of income I have is not passive. Question people when they talk about passive income renting out an apartment building. Find out who is calling the plumber at 3am when a pipe bursts, and ask how passive that is?
Most of the people who do that seriously use a property management company that deals with all of that and finding new tenants when needed. Less profit, but makes it pretty passive if you want it to be.
Website hosting and freelancing web design/dev, at first its not passive, but over time it becomes passive as clients are able to manage their sites themselves after launch.
what kind of savings account requires maintenance and paying always rising local taxes? what percentage of your net worth is tied up in your real estate? what happens when real estate tanks? are you planning to time it and sell at "the top"?