Nu is a Lisp, but it's not the kind of Lisp that a traditional Lisp programmer would write. Rather, Nu exists to serve C programmers, and it does so by building on the Objective-C runtime. Where users of other scripting languages struggle to mix high-performing C code into their applications, Nu users integrate effortlessly when that code is wrapped in Objective-C.
Nu is written in Objective-C and takes full advantage of the Objective-C runtime to create classes, add methods, and interact with compiled objects at runtime. Nu and Objective-C code can be freely mixed, and methods can be called with no regard for the language used to implement them.
Nu's syntax makes it a Lisp, and this allows it to have a powerful macro system and all the flexibility of a language whose code can be easily manipulated as data.
Nu has been used on Mac OS, iOS, and Linux systems. The latest version of Nu (2.0) uses new features in Apple's Objective-C runtimes and runs on Mac OS 10.7 and iOS 4.3 and later.
The Nu implementation is open source and released under the Apache 2.0 License. The primary source repository is on github.
For more about Nu, visit programming.nu or watch this 2007 video in which Tim introduces Nu at the C4 conference in Chicago: