Front load washers....

Thse newer styles stack. Mine had a double sided tape plastic rail on the washer that the feet of the dryer fit in. These things shake though. Put the washer on the bottom! The spin rpm is up to 1800 rpm on the washer, enough to rip delicate clothes per the manual! They really hum and have a certain amount of vibration in there fully suspended baskets. Once you see the amount of water these washers need to wash is incredibly small compared to a top loader. I got my set for $200 and the next week I saw 2 sets for FREE locally when the owner just upgraded to newer models. Keep a look out for them, they are worth it.