I was thinking a few days ago that the more recent challenges are getting more complicated (databases?). Perhaps we should have more skill levels? Like "Intermediate Programming Challenges" and "Advanced Programming Challenges" and so on.
I also came up with an initial Intermediate one. This can actually be done in nine (slightly tricky Python) lines though, so maybe it should be classified with the beginners challenges instead. Well, it was only an idea...
Given a stream of text, find the most common word.
Additional Objectives (optional):
Try to ignore punctuation.
Try to be case insensitive.