Rosetta Code isn't the only project which aims to document tasks in multiple languages. Other projects do exist. Here's a list. Feel free to update and add to it.

Live sites

Multi-Task efforts

Algorithm implementation Wikibooks
Algorithmist - Information about and pseudocode for assorted algorithms; explanations of approaches to contest problems
The Archive of Interesting Code
Beyond Hello World including the CookBook
CategoryInManyProgrammingLanguages on the original wiki
CodeCodex - most similar site to RC when RC went live
Code golf in 104 languages
Common Programming Language Features - C#, C++, and Ruby
Hyperpolyglot
Learn X in Y minutes Take a whirlwind tour of your next favorite language.
LiteratePrograms - Wiki for literate programming samples for various tasks in various languages
Informatyka Wroc Computer Science Wrocław - Algorithms explanations and computer science related articles (Polish language)
le Site du Zero - Algorithmes divers multi-langage (French language)
PHP2Python - specifically for PHP-to-Python comparison for all PHP library functions
php-funcref-in-perl - specifically for PHP-to-Perl comparison for all PHP library functions (text in Japanese)
PLEAC - Programming Language Examples Alike Cookbook
Progopedia - Encyclopedia of Programming Languages (Russian version)
Programming Idioms - Standard ways of coding standard things, in several popular programming languages
Project Euler - Learn programming by solving problems with increased difficulty. After solving a problem the solution can be shared and discussed with others.
Programming Language Comparison
The Computer Language Benchmarks Game
ProgZoo
Scriptometer: measuring the ease of SOP (Script Oriented Programming) of programming languages - includes code examples for common scripting tasks
SwiftAPI
Syntax Across Languages - compares basic instructions, not tasks
Wikipedia:Comparison of programming languages

Specific Tasks

Fibonacci Sequence
FizzBuzz
Is a Cow an Animal?
OO Shape Examples
Spelling Corrector
The Hello World Collection
Wikibooks: List of Hello World programs
doukaku — Communication with subprocesses in Haskell, Python and Ruby
rosetta-cow — cowsay in different programming languages

Humorous

99 Bottles of Beer
Consumers Guide to Programming Languages
Esolangs Wiki
Evolution of a Programmer
Quine
Shooting Yourself in the Foot (PDF) or How to Determine Which Programming Language You're Using
You have two cows

Online Execution

The following sites allow execution of some languages from a web browser.

Site	Languages	Notes
Easylang	EasyLang	IDE, Tutorials
Exercism	70 languages, can browse hundreds of solutions to each exercise.	Pre-specified exercises only
formulae.org	Fōrmulæ	Scripts can be saved/loaded to/from local files
Gambas Playground	Gambas	CLI only
The Go Play Space	Go	Playground with Doc-Ref, Turtle- & Nightmode
The Go Playground	Go	The official Golang playground
Groovy Playground	Groovy	Load save via Github/Gist
Groovy web console	Groovy	Save and share. Runs on Google Cloud (GAE)
Ideone	Bash, C, C++, Go, Haskell, Java, Pascal, Rust, ...	... and many more
Repl.it	Basic, C, C++, Java, JavaScript, PHP, Ruby, ...	... and many more
ScalaFiddle	Scala.JS, a transpiler that translates your Scala code into highly performant and optimised JavaScript.
Scastie	Scastie is an interactive playground for Scala running on a JVM.	Only supports static input
TIO: Try It Online	More than 500 languages	Only supports static input - no interactivity

Algorithms and Data Structures

Algorithm Wiki - interactive demonstrations of many important algorithms
Dictionary of Algorithms and Data Structures - a dictionary of algorithms, algorithmic techniques, data structures, archetypal problems, and related definitions.

MIA

These sites were online at one point, but have since vanished with no forwarding address. If you know of online, existing archived copies, please link to them. Also, Rosetta Code may be interested in hosting static copies of such sites where they're at risk of disappearing. Having such content disappear is counter to the goals of this site.

If a site is listed in this section, but is actually still live, please move it back to the live section.

AlgPedia - The free algorithms encyclopedia
- Home page at Internet Archive
AlgoWiki: The Programmer's Compendium — Site is currently down. Most recent version at the Wayback Machine is from April 24, 2012.
CrossWise
eqcode, the equivalent code wiki
Literate Programs — domain for sale
Mathematica, Maple, Matlab, IDL — Page doesn't exist any more at amath.colorado.edu. Most recent version at the Wayback Machine is from November 29, 2011.
Planet Source Code
Refactory.org — Site is currently down. Most recent version at the Wayback Machine is from October 27, 2011.
SICP in other languages — There is no wiki at codepoetics.com any more. Most recent version at the Wayback Machine is from July 4, 2010.
SICP Wiki — Site is currently down. Most recent version at the Wayback Machine is from July 25, 2013.
ROT13 — Page is currently down. Most recent version at the Wayback Machine is from June 15, 2013.
Windows Phone 7 Mapping - mapping to Windows Phone 7 API, from the APIs of iPhone, Android, and Qt
Hanoimania
Numerical Recipes Books On-Line, statistical programs in Fortran and C, dead-tree books now available as free e-Books
Real-Time Software Rendering
TPK algorithm in several languages
Infinite loops and recursions
Kiloseconds

CodeUU

CodeUU

Sent an email to the domain-by-proxy contact address, to try to get in touch with the owner. --Michael Mol 17:04, 14 January 2011 (UTC)

NoCruft

NoCruft Code Wiki

Sent an email to the domain owner. --Michael Mol 17:05, 14 January 2011 (UTC)

It's gone...

The Internet Archive crawled the site 3 times in April of 2008. http://wayback.archive.org/web/*/http://www.nocruft.com/wiki/index.php/Main_Page Unfortunately there doesn't seem to have been much content on the site at the time.