Google Chromecast Audio: Stream your favorite music with Google Chromecast Audio. Simply plug into your speaker's auxiliary input and connect to your Wi-Fi network. Then just tap the Cast button from your favorite apps on your phone, tablet or computer to cast songs, radio stations and podcasts to your speaker.
Picked up three of these babies for around $100 total. Setup was super easy using the iphone chrome cast app. Can stream NPR, Spotify and Google music straight from the internet - frees up phone to make calls, text, etc. Can also use Google Cast extension for chrome to cast audio to the device. Has analog and digital optical out (same as on Apple Airport Express). Sound is very good (make sure to select the HDR audio in settings). Currently, software does not allow for "multicasting" or selecting multiple devices to play same music, but it is promised in a future update. Does not support direct streaming of Apple Music. Overall - a great way to turn a set of speakers into a makeshift (and cheap) Sonos substitute.