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
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 93 languages
Common Programming Language Features - C#, C++, and Ruby
CrossWise
eqcode, the equivalent code wiki
Hyperpolyglot
Langref.org
Literate Programs
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)
Planet Source Code (Warning! Popups!)
PLEAC - Programming Language Examples Alike Cookbook
Progopedia - Encyclopedia of Programming Languages (Russian version)
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
Rosetta Stone
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
Windows Phone 7 Mapping - mapping to Windows Phone 7 API, from the APIs of iPhone, Android, and Qt

Specific Tasks

Fibonacci Sequence
Hanoimania
Is a Cow an Animal?
Numerical Recipes Books On-Line, statistical programs in Fortran and C, dead-tree books now available as free e-Books
OO Shape Examples
Real-Time Software Rendering
ROT13
Spelling Corrector
The Hello World Collection
TPK algorithm in several languages
Wikibooks: List of Hello World programs
doukaku — Communication with subprocesses in Haskell, Python and Ruby

Humorous

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

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.

AlgoWiki: The Programmer's Compendium — Site is currently down. Most recent version at the Wayback Machine is from April 24, 2012.
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.
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.

Algorithm Wiki

Algorithm wiki

I emailed the domain owner, asking for news, and offering to host an archival static copy. --Michael Mol 16:59, 14 January 2011 (UTC)

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.