Choice of development environment is often a choice that is preordained, either by your job or by the project. When I get home and just want to explore on my own, I've come to appreciate things a particular way.
I use the following for my personal projects:
Host OS - Windows 7
One of the best Windows versions in my opinion, appears better than the past. This is augmented by Microsoft's free virus protection software, Microsoft Security Essentials. The UI is very responsive with a minimalist system tray. I take the minimalism a step farther by removing all icons, including the Recycle Bin, from the desktop. The search bar under the 'Start' button is also so quick that I don't ever navigate the 'All Programs' start menu manually. To top it off PowerShell gives you a terminal with a lot of the Unix goodness plus objects, to make scripting much more palatable in Windows.
Virtualization Tool - Virtualbox
If you're going to go exploring on your personal machine, you need a safety net. If you spread out all your tools on your Host OS, for multiple technology stacks, you'll end up with a slow buggy mess. Virtualization lets you divy stacks between environments, so no one steps on anyone else's toes. You screwed up an install? Nuke the OS. It isn't like it will prevent you from using your Host OS.
Virtual Machine OS - Ubuntu
I love Windows, but it really doesn't do everything I need it to. Actually, outside of being a consumer of programs or developing against the CLR, Windows still is pretty bad. Too many open source projects have a Linux first mentality. Also, I'm in it to learn something when I'm playing around at home. Linux is a foreign land to me still in a lot of ways, so I still have plenty to learn. It also doesn't hurt that it is free (as in beer).
Version Control - Git
Version control is a godsend when exploring a new technology stack. Not only can you save your work, but with Github you have a way to present it to the world. It also is the method of choice for open source projects, which I want to contribute more to. Also I use Mercurial at work and Git is just different enough to be a good learning target.
Text Editor - Vim
Beautiful. Powerful. Arcane. More learning that I'll probably be able to do in a lifetime. Vim proficiency is a life goal and my free time is when I can justify looking up a new text editor command every ten minutes. Also, it forces me to think in Regex.
Web Browser - Chrome
I use it everyday, at work and home, and I still have a lot to learn. The Chrome developer tools have depth that I have not begun to fathom. It makes browsing and developing web applications a pleasure.