Simple calculator using eval
I do not know Ruby, but from my Python knowledge, it is not good to use eval in a real program.  When you use eval, it executes the code directly, so accidental/experimental input can have unwanted results, or even destroy the system.  Also, this has the potential for a hacker who has limited access to the system to run this program and do whatever they want as long as they have access to it, instead of cracking in deeper and doing whatever they are trying to do.

See this question for a detail discussion about eval in Ruby.

This question is about python, but also has good points.