Justin Williams over at Mac Zealots posted an article back in April about the various development technologies that ship in OS X Tiger, called Mac OS X Tiger: For Developers. He gives a great overview of the platform and why it is, indeed, a developer’s paradise.
In addition to the tools and technologies that come with OS X, I also use Ximian’s Mono Project for C# development and Java 1.5 Tiger (OS X still ships with 1.4.2, but you can get 1.5 from apple here).
As for development environments, I’m primarily using Apple’s XCode 2 nowadays. C# development is best done in using MonoDevelop, which runs using GTK# under X11. Installation can be a little tricky, Brian Jepson provides instructions here. For Java development, I love JetBrain’s IDEA, but it’s not free (as in speech or beer); but for it’s features and stability, I think it’s worth the price; I own a license for IDEA 4.5, and IDEA 5.0 was just recently released. Of course for you Unix freaks there’s always trusty vi and emacs.
For you web developers, OS X ships with Apache 1.3.3 with mod_php (version 4), mod_perl and mod_dav. You can get mod php 5 from Mark Liyanage as well. You can install Apache 2 using a package management tool like Fink. Tomcat 5.0 and 5.5 run on OS X as well under Java 1.4.2 and 1.5 respecitvely. For something more robust in the Java world, there’s also JBoss.
So all in all, you really can’t miss as a developer on the OS X platform; and since it’s a BSD at heart you’ll have virtually no work to do in porting existing projects… It’s what drove me to make the switch back in 2002, and I’ve never looked back.