Mac OS X combines the best of Free Software and Proprietary Software, and in this post I'll try to show why I believe it is the best developer-friendly OS, and try to fend off the myth about OS X being useless.
• Cocoa and Objective-C : When it comes to API and programming language nothing beats this combo. It's better thought out and offers virtually everything, except for the times when you need to dive into Carbon; read this article, for a better indepth explanation.
• Xcode : Certainly not the best IDE out there; but being the official IDE of the Mac OS, and being capable of providing the developer everything he/she needs in a single version (no basic and professional nonsense) for free more than makes up for it minimum shortcomings.
• MacPorts : This project enables developers and users to easily compile and install various Free and Open Source programs and libraries, bringing you the best the FOSS community has to offer.
• Unix : Mac OS X is a full-fledged SUS-certified and POSIX-complaint Unix, and version 10.5 (Leopard) comes with X11 and the popular BASH pre-installed.
This list is obviously non-exclusive, but add this to the ease-of-use, simplicity of the Mac, and growing market share; and you get a very strong argument for development on the Mac.