We use these (from Home Depot):
We never used them with our old Maytag front loaders, and it wasn't a problem. We just bought a new LG set, and figured we would go ahead and use these for good measure. We used the washer a few times before we got them, and while it wasn't bad, there was a bit of shaking - enough to knock a bottle of detergent off the top. It seems less with the pads installed, but they don't eliminate all vibration completely.
We put them under both the washer and dryer to maintain the same height. ($30 per set of four.) They are about an inch and a half thick and quite heavy - it's a very dense rubber. They off-gassed quite a bit for the first couple of weeks with a very strong rubber smell, but the smell has now mostly gone away.