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.
