I found this in ebay. Is it good or bad?
No, that's a current sensor (not voltage).
And I'd be careful about buying stuff on eBay where there's no link to manufacturer's data sheet. (Or where the datasheet is incomplete.) And, I'd be very careful about buying anything from an unknown supplier/manufacturer that you're going to connect to the power line!!! (The current sensor doesn't have a direct connection to the line voltage but a transformer does.)
Probably the safest thing is to get a [u]step-down transformer[/u].
Of course, you'll need a voltage divider and then you'll either need to [u]bias[/u] the input (and then subtract-out the bias in software) or rectify it to get rid of the negative voltage.
From there you have a couple of choices... You can find/measure the peaks and calculate RMS, or calculate the average and convert that to RMS. (You can also "properly" calculate RMS, but that's not necessary because with sine waves the relationships are known.)
The voltage out of a transformer isn't "perfect" but with a constant load it is a constant ratio of the input voltage. So, measure the 220VAC voltage with your multimeter and measure the transformer's output to calibrate your setup (in software).
...measuring only DC voltage (0v~50v, total 6 input)...
...(really simply, analogue voltage -> diode -> resistor -> arduino mega)
Is 50V a typo? Because that won't work. You need a [u]voltage divider[/u] (2 resistors) if you want to go over +5V.
A single resistor won't do anything unless greater than 1M Ohm or more. And if the resistor is large enough to do anything it will have unpredictable effects and it will make it more prone to noise pick-up. The diode will introduce a ~0.7V drop which makes it impossible to measure voltages lower than that, and it will introduce a 0.7V error (which you can subtract-out).
I'm worrying about noise from AC.
At this present, my data logger, even DC, shows values with noise from somewhere.
Of course, my data logger has no device for reducing noise.
The noise could be in the actual voltage you're measuring or it could be noise in the Arduino's power supply, which is the default ADC reference. Switching to the 1.1V reference will eliminate noise from the ADC reference. (You'd have the keep the analog input below 1.1V.)
Also, take a look at the [u]Smoothing Example[/u]. You can also use some "fuzzy logic" to throw-away any readings that seem way off.