One of the funnest ways to learn new languages and sharpen your coding skills is to do coding challenges and puzzles. There are even competitions arranged around solving computer programming challenges and puzzles. Also, don’t be surprised if some of the problems listed show up in job interviews. This is a common way to filter out good programmers from not-so-good programmers when interviewing.
Here’s a list of nice code challenge sites. If you have a favorite I don’t have listed, please contact me and let me know.
Code challenges and puzzles
Rosalind (bioinformatics problems)
Timus Online Judge
L-99 (Lisp-like languages … like Clojure)
Peking University POJ
HIT Online Judge