Remedial English

Evil

English isn't my first language so I sometimes have trouble with its nuances. I am a semi-regular user of FriendFeed, a service where you can read people's comments on various topics and threads. Semi-regularly, I will come across a comment like this:

"I'm just sayin'..."

or a variation:

"I'm not sayin', but just sayin'..."

What does this mean??!! I don't even know what it means and everytime I see it, it bothers me. It just seems like a douchy thing to write. Does it mean any of the following:

"I told you so."

"I am telling you so... even before the fact."

"I don't want to say outright that you're a dumbass, so I will just imply it."

Anyone know?