A Really Useful Box2D Editor
You're a game developer. Your next project will have Box2D-powered physics, and it'll be awesome. So you wrestle with installers, build systems, possibly with compiling the library itself, and then get a rectangle moving around on-screen. Now what? If your game is non-trivial, you could have dozens of different types of objects. You're not going to have a good time describing them in code, as you'd have to recompile every time your designer (which could be you) blows his/her nose. You could come up with your own loader, but then you'd have to come up with your own file format, too, and that's time not spent actually making your game!
Enter R.U.B.E., the Really Useful Box2D Editor. R.U.B.E. is a level designer for virtually any game that uses the Box2D physics engine. You can use it to visually create a wide variety of collision shapes, place them on-screen, and edit their properties. R.U.B.E. also boasts a comprehensive scripting system, support for nearly all Box2D constructs, and extensive customizability.
I use R.U.B.E. on Ubuntu 14.04 and a reasonably beefy 2011-era Samsung laptop. R.U.B.E. is available for Windows, Mac, and Linux. The Mac version, however, does not officially support 64-bit computers; it might or might not work. Likewise for any Linux distribution other than Ubuntu. When in doubt, see if the trial version works. I'm using R.U.B.E. to develop a top-down shooter written in Java and libGDX.
The user interface for a program like this should aim to minimize the barrier between the designer's ideas and his/her implementation. Fortunately, R.U.B.E.'s editor does a decent enough job of that. The controls for the editor itself are a bit clunky, relying on keyboard shortcuts to perform basic geometric transformations (e.g. pressing S to scale an object, as opposed to just grabbing a handle like in Unity or Inkscape). A bit annoying, but you'll get used to it. Other features, like the ability to position the menus freely, or the ability to copy objects to other programs as plain text, or even to run the Box2D engine to see how objects move in the world, are very much welcome. I don't think you'll be wrestling with the program too much.
My limited experience with RubeScript, the built-in scripting engine derived from AngelScript, was a pleasant one. The language itself is nothing spectacular, preferring simplicity for programmers and non-programmers alike over flashy features. Better documentation for the built-in types (strings, arrays, etc.) and code completion would've been nice, at least.
What really makes RubeScript stand out is its thorough functionality. Almost all of the built-in editor actions--adding or deleting bodies and joints, smoothing or removing vertices--are actually implemented in RubeScript, and can be viewed in R.U.B.E.'s installation. If the developer himself can write the meat of R.U.B.E. in its own scripting system, I'm sure you can get something out of it, too. Tools like this are why game designers benefit from some programming skills.
Minimal. Most available alternatives lack some of R.U.B.E.'s most defining features. These other editors are only designed for creating simple game objects (such as PhysicsEditor, which doesn't support joints), or lack adequate import/export facilities (e.g. BoxCAD, whose output requires ActionScript 3.0 to be used). Other more generic solutions such as Tiled or even Inkscape might suit your needs if you can write a loader to convert their output to Box2D worlds. I don't feel that most of these other alternatives fill the need for a 2D physics world editor, though for very small games you may disagree.
The Surrounding Ecosystem
I have nothing but good things to say here. The official documentation for R.U.B.E. is excellent. Even if you ignore the built-in manual (which is very detailed and contains plenty of images), there's a wealth of resources available on the official website, including sample projects and video tutorials.
But perhaps best of all, Chris Campbell (the developer) actually gives a damn about you. He reads the forum and any e-mail he gets. It's easy to report bugs or offer feedback from within R.U.B.E. itself. I can send a hastily-written bug report or feature request through through R.U.B.E. and continue working before my concentration wavers. You'll actually get a response from Chris within a reasonable time frame, and he'll make sure you get the best out of R.U.B.E. you possibly can. He's helped me numerous times with reporting bugs, feature requests, and even general Box2D questions.
The main problem with R.U.B.E., as I see it, is that your game's technologies will be limited to whatever has a R.U.B.E. loader and a Box2D port available. This issue isn't exclusive to R.U.B.E.--it's inherent in any technology that's not as mainstream as, say, PNGs or WAVs. While R.U.B.E. does have loaders available for many popular game frameworks and programming languages, if none of them suit your needs you'll either have to reconsider your toolkit or write your own loader.
The Bottom Line
A designer alone may not have much use for R.U.B.E., because odds are if he/she is using a prefab game engine like Game Maker or Unity he/she'll be using its respective built-in editor. But for programmers or designers working with programmers, R.U.B.E. is a solid investment. R.U.B.E. is flexible enough to play well with any custom game engine, yet simple enough to make entertaining game worlds with ease. I wholeheartedly recommend taking the $30 plunge.