Raspberry Pi: Probably THE embedded plamform at the consumer level, although can be used for production purposes.
Onion Omega2: A great product if you need an “Arduino” like device but with the power of Linux and wifi connectivity. At a base price of $9, really hard to beat!
Soekris: Looks like a good selection of embedded x86 compatible hardware. Recommended by a consultant years ago.
RouterBoard: Provider hardware for MikroTik RouterOS, but could potentially install Linux