Coco A CoffeeScript dialect that aims to be more radical and practical, also acts as a test bed for features that get imported in CoffeeScript.
Parsec CoffeeScript CS based on parser combinators. The project's aim is to add static metaprogramming (i.e. macros + syntax extensibility) to Coffee Script (CS), similar to how Metalua adds such features to Lua. The resulting compiler, once merged with the official compiler, should be usable as a drop-in replacement for it.
Contracts.coffee A dialect of CoffeeScript that adds built-in support for contracts.
Uberscript, a CoffeeScript fork that adds type annotations which are compiled to Google closure compiler type annotation comments.
ToffeeScript A dialect of CoffeeScript that support Asynchronous Syntax, Symbol and Regexp operator =~
Caffeine A dialect of CoffeeScript that support packages and classes import, useful for browser applications
Friends (philosophically related)
Haxe compiles to several platforms (C++, Flash, JS, Neko, PHP).
Fantom Evolutionary object-oriented language emphasizing succinct and effective APIs (JVM, CLR, JS).
Tierless languages (produce both client & server)
Fun A programming language for realtime webapps - compiles to client-side and server-side JS.