As for anything, there are alternatives. However, the Teensy is NOT cheap in comparison, and would be $16 for the 2K version or $24 for the 8K version (more in the UK, no doubt). The ATmega328 is only a couple of dollars/pounds, plus it is well-used and understood on this forum. The mega328 chip is also easily available as a simple DIP package so any hobbyist can use it. Actually, given the size, you can squeeze the two chips needed for this version into a similar size factor that Teensy supports. Could also go other ARM chips etc. etc. etc... but that's not the purpose of this forum or thread 
Grant