In the spirit of Tuesday App Roundup and Doug Hellmann's Python Module of the Week, I want to start a pan-language weekly library round-up.


1. Please form your responses with: [Language] Library Name that links to the library's homepage or archive (PiPy/CRAN/CTAN/etc.) listing - what the library does and why you're recommending it.
2. ONE library only please. Do you like a few? Come back next week and submit your 2nd.
3. You can choose whatever language you like: Python, Javascript, Java, Prolog, Haskell, SQL, x86 assembly, Piet, any of the zillions not listed here, don't care.



[Python] docopt - It generates a commandline parser based on a properly formatted USAGE string, inherently keeping the help message in sync with the commandline arguments. There are also Ruby, CoffeeScript/JavaScript, PHP, Bash, and C#/.Net implementations.


Image source: Dan FunderBurgh via