CocoaTech has released PathFinder 4, the 'Finder replacement' on Mac OS X (which is both hard since the Finder is baked into the OS and included, but easy since the Finder doesn't really provide much actual UI). I haven't used PathFinder much before, since it had publicly stangated on v3 and I am happy enough with the Finder and the awesome ability to press --G to bring up a tab-completing text entry for going to locations.

But what really impressed me was their new Open Source projects. Their open-sourcing their plugin-in interface, their internal and powerful frameworks, and CocoaTechTerminal which allows developers to put a terminal within an NSView in their applications.

A company with solid code, releasing parts of their code-base in open-source is unadulterated awesome and much to be applauded. I'm downloading PathFinder 4 just because of their open-source coolness and will give it a try and may purchase it.

Since I'm on the topic of people making very developer friendly add-ons, I recently came across John R Change Contributed projects, which include a better NSStream, a case-insensitive NSDictionary, a category of NSString that adds support for matching regular expressions, and other Cocoa add-ons and classes.

There are many, many more libraries out there. Just watch CocoaDev, a developer Wiki that is active (if sometimes chaotic) documentation, discussion, and resources on Cocoa libraries.