Someone recently asked in the Google Plus Ruby on Rails community: Which platform would be the best to use for Rails? My answer got so long, and is so applicable to working in most other languages, that I decided to turn it into a blog post, so here it is, with a few minor edits.
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Basically, any reasonably popular platform EXCEPT Windows.
Windows (ignoring the Server variants) is made for the desktops of non-technical people (and people developing software specifically for Windows). It comes with essentially zero development tools, and you have to pay for most of the serious ones, especially for the MS stack. You can kinda-sorta fake a lot of Unix by installing Cygwin, but that's just a kluge and falls way short.
Linux and other Unix variants such as BSD, Solaris, etc. are made for servers, and the desktops of VERY technical people. They come with (optional on installation) lots of serious development tools, with many more easily available, most of them free. Of these OSes, Solaris is pretty much dead, and BSD is very rare outside the server room (so there's nowhere near as many resources for help and education), and the others except Linux have very tiny market share, so let's focus on Linux. However, Linux has a bad reputation for requiring a lot of tweaking to get it to work reasonably well, especially if you're using hardware that is in any way not absolutely standard, such as a graphics card from within the past year or a maker that's not one of the top three. This was reality, and why I switched to a Mac, in 2004, but I've heard Linux has gotten a LOT better about this since then, especially the Ubuntu "distro" ("what's a distro" is a whole 'nother question!), which (I've heard) places great emphasis on working right out of the box. Linux is also free (though you can buy boxed sets with docs, support, and so on), and generally efficient enough to make good use of older PCs that won't run well under recent versions of Windows.
A Mac is a reasonable compromise, at least as of when OSX first came out. (Before then, it was aimed mainly at graphics people, like artists and people who put together print newsletters and magazines.) The tooling situation is similar to Unix, except that it doesn't come with quite so many, and usually older versions. It also doesn't require anywhere near as much tweaking as Linux does, because you're running it on exactly the hardware it was designed for. It's even more consistent in its UI behavior and look-and-feel than Windows, and about as easy to understand -- but it's different, so you'll have a lot to "unlearn" if the Windows way of doing things is deeply ingrained in your habits. It also used to be much more stable and secure than Windows, but MS has made great strides in their security and stability, catching up and, depending how you measure things, possibly surpassing OSX's security (not sure about stability). On the other claw, it's a good bit more expensive than a bog-standard Windows box, never mind Linux, but frankly, they're so good that putting together a Windows PC with the same performance will usually cost about just as much, even before factoring in the cost of serious dev tools, OS upgrades (usually dirt-cheap or even free on a Mac), etc. You can get some good bargains on a used Mac, one or two generations old; ask a Mac-using friend to sell (or even give, if you're lucky!) you one of his old castoffs, or take your chances on eBay.