The Echo Dot ($90) is pretty much the top sliced off the regular Echo, to be used with another speaker.
Echo Dot article on The Verge: http://www.theverge.com/2016/3/3/11148532/amazon-echo-dot-limited-availability-echo-firetv-alexa
Amazon listing: http://www.amazon.com/b/?node=14047587011
Annoyingly only available to order via Alexa at this point. See comments for a workaround though!
They have also released the Echo Tap ($130) a portable Bluetooth speaker (although you have to press a button to activate Alexa functionality) - http://www.theverge.com/2016/3/3/11148776/amazon-echo-tap-speaker-price-availability-photos-video