Java
Java is both a strong development environment for the Mac in terms of the tools available and also a way for developers to bring their applications to the Mac platform. In terms of IDEs you are spoilt for choice with popular tools like NetBeans and Eclipse vying for place on your Desktop. Many popular cross-platform applications like Azureus and Oracle SQL Developer found their way to the Mac thanks to it's strong Java support, which makes it easier for developers to port their Java applications to the Mac.

One thing to note for Java Developers is that all Java updates for the Mac are published by Apple and not Sun, as it does for other platforms. This means that often there is a slight lag between the time a major Java version is available for, say, Windows and Mac.

Unix
Mac OS X is a full POSIX complaint system, meaning you can do on on the Mac pretty much whatever it is that you can do on your favourite Unix/ Linux flavour. So go on use your favourite *nix tools (awk, sed etc.) and write shell scripts for your favourite shell - Korn (ksh), Bourne (bash) etc. You have the advantage of using powerful graphical text editors like TextMate, BBEdit, TextWrangler and others for writing/ editing your scripts if vi and emacs give you the creeps!

While not restricted to the *nix platform, both Perl and Python have strong *nix underlying philosphies. Mac OS X comes pre-installed with both Python and Perl. Affrus is a popular Perl IDE offering integrated and debugging support.

Oracle
Oracle developers can get the Oracle client that allows them to connect to Oracle databases from their Mac. Or you can make your Mac into a database server by installing the Oracle server for Mac. Though there isn't quite a TOAD (a popular tool on the Windows platform) equivalent for Mac in terms of the sheer volume of features that TOAD provides, Oracle's very own SQL Developer and a third-party Mac native application called SQLGrinder are more than able deputies.

C/ C++
Mac OS X developer tools include gcc which you can club with your favourite IDE/ text editor to get a first class C/ C++ development experience. As far as libraries and code portability are concerned, your development experience would be comparable to that of developing on any other *nix flavour.

Windows
I am sure this is one sub-category that you never expected to see here. Truth is, however, you can use .NET Studio and whatever else it is that you use on your Windows machine, exactly the same way on your Mac to develop native Windows apps! This thanks to VMware Fusion/ Parallels that allow you to work seamlessly with Windows apps within Mac OS X, or Apple's Boot Camp that allows you to dual-boot between Windows and Mac OS X.

iPhone
Though you can develop web applications or widgets for the iPhone on any platform, if you are planning to build "real" applications that use the underlying OS technologies, you'll need to develop via the iPhone SDK. The iPhone SDK, as of writing of this article, requires Mac OS X Leopard and does not run on Windows or any other platform. As part of the SDK iPhone developers gets the same set of tools as their Cocoa counterparts - Xcode, Interface Builder and Instruments - as well as an iPhone Simulator, to see how your application would actually behave on the iPhone.

About the author
(Kunal is a telecom billing consultant by day and a Mac nerd night. His CV reads 6 years of experience as an IT consultant in various mish mashes of roles and business domains he's stumbled upon one after the other. His love affair with computers started way back when MSDOS and BASIC were the norm and it has only grown stronger since then. He lives in Pune with his wife and 2 Macs.)