ND filters. Why variable?
ISO isn't magic. When you turn up the iso you're increasing the gain on an amplifier that is amplifying an analogue signal – the light hitting the sensor – and when you amplify a signal you amplify the noise. Remember that opening up a stop is a doubling the light intensity. To get ten stops of gain on a camera with native iso of, say 200, you'd have to go to iso 204800. So whacking a 10 stop ND (also known as a 'Black Filter' because you could literally use it as welding goggles) on the front would mean that all you'd get is noise soup. 

A 10 stop ND is really only useful for the harshest sunlight with a fast lens with a wide open aperture, or for still and time-lapse photography if you want extremely long exposures. It's certainly not the first filter I would buy as a beginner.

A Variable ND filter allows you to change the exposure in bright conditions without affecting other picture characteristics. Increasing the f-stop increases your depth of field, which may be undesirable, and increasing the shutter speed (i.e. reducing the exposure time) reduces motion blur, which may mean more strob-y motion (as you often see in sports coverage), again, this may not be desirable.

So a variable ND changes the exposure without changing the feel of the shot as the other ways of reducing exposure do. One thing to be aware of is that some variable ND filters do cause colour shifts across their range. So if you go for a bargain basement one off ebay buy a Macbeth card at the same time because you will have to compensate in post.