Sometimes it's nice to open an interpreter, type out some code and see if it's working, rather than having to compile something large like an Xcode project just to quickly test something. Does an Objective-C Interpreter exist for the Mac, or am I out of luck?
UPDATE: There's a paid program in the App Store called CodeRunner, which lets you run Obj-C quickly (and several other languages). It's not an interpreter, but works pretty well for testing out ideas quickly: http://itunes.apple.com/us/app/coderunner/id433335799?mt=12
You can use F-Script, which is Smalltalk-based, but lets you interface with Cocoa. You can also embed it in your application, to let you inspect views and other things at runtime. It's quite useful. Edit: as pointed out in the comments, there are other great language bridges available, like PyObjC, RubyCocoa/MacRuby, Nu, etc. CINT is an example of a C interpreter, but not Obj-C.
As for an actual Objective-C interpreter, I don't think one really exists. But it's not too hard to set up a "Command Line Tool" project in Xcode, which gives you a really simple main.m file to try something in.