Want to find all live instances of a class at runtime while debugging? Richard Heard has put together this small library that will allow you to do exactly that. It compiles into your app but is mostly useful from within LLDB for live investigation of objects. Also, this is the best named project I've seen in a while.
Update: Thanks to Krzysztof Zabłocki for pointing out that this code seems to have been taken from another project, Cycript by Jay Freeman. While the code remains useful, the disregard for the original code license is not good enough. Apologies, I would not have linked to it if I had noticed this.