The one-line blurb sounds a lot like my project, Cycript, so I figure I'll mention it here. I've been working on a new website recently with documentation, and have a new release coming very soon with iOS 7 and ARM64 support (along with a ton of new features) as well. (All of this likely to be released quite soon, but many developers have been actively using Cycript since I first released it in 2009 regardless: it is one of the more important tools for iOS introspection in the jailbreak community.)

Cycript is an Objective-C/JavaScript bridge that allows you to inject code into native applications on Mac OS X or iOS (either using a jailbroken device, into the simulator, or using an SDK to embed into your application). (I do not currently support Windows, or runtimes like .NET.) It features a highly-interactive REPL with runtime grammar-assisted tab completion and live syntax highlighting. Users type am enhanced JavaScript syntax that includes a number of features from C and Objective-C.

Here is a link to a talk I gave at 360|iDev demonstrating its usage, as well as how to download the current beta version for Mac OS X 10.9 and the iOS 7 version of Xcode. I am also here linking to the Cycript website (which still sucks, but gets the gist across of what Cycript is and how one interacts with it; the talk is better, though, and more up-to-date) as well as the iPhone Dev Wiki (which has a lot of third-party documentation).

http://www.youtube.com/watch?v=5d1cK0nq4GY

http://www.cycript.org/

http://iphonedevwiki.net/index.php/Cycript

Cycript lets you swap out implementations of native code in an Objective-C application, and can do do at the C function level quite easily when combined with Substrate (which is very easy to call out to, as Cycript has quite sophisticated support for FFI due to the large amounts of C-specific syntax and bridging it has; the next release is going to be even better at this, with the syntax now supporting type signatures specified using C-style type signatures, and with C++11 lambda syntax for more direct construction).

To compare, Frida looks a little more like a scriptable debugger, whereas Cycript is designed more to interact with applications as they are being used. With Frida, you use a Python API to inject JavaScript snippets into the other app, and then do IPC back and forth with your code. Cycript instead gives you an interactive console to a JavaScript environment running in the other application, with IPC implicit in the REPL. (FWIW, there are advantages to the kind of approach Frida has, and adding features to Cycript to make it a little more like a debugger have been on my todo list.)