Here's another one from Programming Praxis. It's an RPN (reverse Polish calculator) problem that I've chosen to implement in Ruby. The text from Praxis is the header comment of the program.

The first thing we do is open up the Array class and alias last with a new method called peek which returns the the last element of the array. Obviously, we could just use last, but the "norm" for stacks is peek. Next we actually create the stack using an Array. We could probably create our own stack with only the appropriate methods, but that's probably overkill for this little project (as I suppose you could argue creating peek is). Next, we'll just loop forever, printing a prompt,reading a line of text, and then processing it. The processing consists of taking a line apart and grabbing either a number, the first regular expression, or an operator, the second. For a number, we simply push it on the stack. For an operator, we pop the top two items off the stack and perform the operation on them. Then we push the result back on the stack. Finally, we strip whatever we just found, either number or operator, off the line and continue. When we finish processing the line, we print the top of stack and then continue on to the next line.