Justin Williams over at Mac Zealots posted an article back in April about the various development technologies that ship in OS X Tiger, called Max 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.

Most popular open source databases are available for OS X as well, Mark Liyanage has assembled native OS X installation packages for MySQL and PostgreSQL

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.

Finally, most (if not all) of your favorite project management tools work for OS X as well, including Ant, make, Maven, CVS, and Subversion.

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.