There’s a decent list of open source Mac projects on CocoaDev: http://cocoadev.com/CocoaOpen
Not all of the projects are still active, but a decent number of them are. I particularly recommend:
- BWToolkit – http://bitbucket.org/bwalkin/bwtoolkit/
- BGHUDAppKit – http://www.binarymethod.com/bghudappkit/
- CHDataStructures – http://cocoaheads.byu.edu/code/CHDataStructures
- MGTwitterEngine – http://mattgemmell.com/source
- Sparkle – http://code.google.com/p/sparkleplus/
- Colloquy – http://colloquy.info/project/wiki/Source%20Code
- CocoaRest – http://github.com/sdegutis/CocoaREST
- CorePlot – http://code.google.com/p/core-plot/