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.
http://webchat.freenode.net/?channels=%23%23passiveincome&ui...
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?
A few hours a month may be low work residual income.. but not passive.