I have been using my new Mac Mini for less than 24 hours and it already looks like this:

In the screenshot I have VMWare Fusion with Unity enabled so that I have the Windows Vista Start menu (I can toggle off the Start menu's visibility from VMWare itself) and Internet Explorer 7. (I also have Visual Studio 2008 installed in that virtual machine). Next to Internet Explorer on the left is Finder which is showing a bunch of the apps I have installed, including most of the stuff at http://www.opensourcemac.org/. On the right I have MonoDevelop where I can write C# or VB.NET applications for the Mac, for Linux, or for Windows. And of course, down below I have the Dock popped up because that's where my arrow actually is.

I also, obviously, have an Ubuntu VM I can fire up any time I want if I want to test something in Linux. 

Mac OS X 10.5 (Leopard) comes with native X11, not out of the box but with the installer CD, and it's the first OS X build to do so (previous versions used or required XFree86).

This point in time is a particularly intriguing milestone date for the alignment of the moons and stars for blissful cross-platform development using the Mac as a central hub of all things wonderful:

• X11 on Mac OS X 10.5
• MonoDevelop 1.0 is generally gold (released, it's very nice)
• System.Windows.Forms in Mono is API-complete
• VMWare Fusion's Unity feature delivers jaw-dropping, seamless windowing integration between Windows XP / Vista and Mac OS X. And to make things even more wonderful, VMWare Fusion 2, which comes with experimental DirectX 9 support, will be a free upgrade.
• For game developers, the Unity game engine is a really nice cross-platform game engine and development toolset. I have a couple buddies I'll be joining up with to help them make cross-platform games, something I always wanted to do. This as opposed to XNA, which doesn't seem to know entirely what it's doing and comes with a community framework that's chock full of vaporware. (But then, I still greatly admire XNA and hope to tackle XNA projects soon.)
• The hackable iPhone (which I also got this week, hacked, and SSH'd into with rediculous ease), which when supplemented with the BSD core, is an amazing piece of geek gadgetry that can enable anyone to write mobile applications using open-source tools (I'd like to see Mono running on it). The amount of quality software written for the hacked iPhone is staggering, about as impressive as the amount of open source software written for the Mac itself. Judging by the quantity of cool installable software, I had no idea how commonplace hacked iPhones were.
• Meanwhile, for legit game development, the Unity 3D game engine now supports the iPhone and iPod Touch (so that's where XNA got the Zune support idea!) and the iPhone SDK is no longer just a bunch of CSS hacks for Safari but actually binary compile tools.