I've been wanting to write an OSX app and get into Xcode recently, so I decided to make a nice Cocoa-based 7-Zip for Mac.
It was hard to understand the black magic behind InterfaceBuilder, but Apple has a bunch of nice examples built in to their help system which helped a lot.
Objective-C is weird at first. You dont usually need to implement an interface (like c#), but if you name your methods the correct name, they just magically get called by the runtime. Makes discovering how to do things a google/stackoverflow exercise, rather than inside your IDE.
This is v0.1. There is no support yet for creating archives, zip files are set to open with 7-Zip Mac by default and there are no preferences. But its useful already I think.