Real Basic is a good one for SUPER-easy development. The only problem is that its not free and not good for all development projects.
I think another good alternative would be Java with JBuilder Personal Edition. JBuilder has a GUI building interface which makes building an interface easy and the Personal edition is free. Also, Java is a pretty straightforward language.
The third option would be Cocoa with Objective-C. Again, you can use Apple's free tools to build an interface. This might be the hardest (relatively, of course) of the three options to get used to. Its still very easy, though.
There are also tons of other little languages out there for OS X. Python comes to mind as an easy little language but its without robust methods for getting GUIs up.
If you want cheap and free, you can sign up to be an Apple Developer and download all the Java and interface utilities directly from Apple. But it might be a bit difficult to get started, but you should play with it anyway.