Generally said as "Not f'nuthin', but...."
NYC/East Coast slang of the caucasian working class (particularly Italians)...
"Not for nothing" is used to soften the blow of something that would normally be offensive or come on too strong.
For example, if your friend showed up wearing purple and yellow striped pants to a party and asked you how he looked, you could respond, "well, not for nothing, Jimmy, but you look like a gay, retarded zebra." Or if you want to try out for a musical but you can't sing to save your life, your friend might tell you, "hey, not for nothing, but you suck at singing and you're not gonna get that part." Things of that nature.
It can also imply consequences, for good or bad. "Not for nothing has Shaq played basketball all those years." Or, in a negative way, "Not for nothing has David Duke earned a reputation for bigotry."
It does not mean "for what it's worth," a common mistake. To see why this does not work, replace the words: "For what it's worth is Paris called the City of Lights."
There is a similar Italian phrase which is not used in the same manner in Italian, "non per niente."
It is not for nothing you are named Ransom. -C.S. Lewis
Not for nothing is the Urban Dictionary known as the repository of online slang.
Not for nothing do Red Cross volunteers offer aid around the world.