This is my sandbox campaign for Planescape. It is pretty much a brainstorm and a collection of useful material I made for DMing Planescape adventures in various systems. There is a lot of story hooks, NPCs, encounters, organizations, places and etc that I prepared over the years, and a lot more I prepared for this campaign. This is here most to help me organize stuff than for others to see, but feel free to look around.
So in short: I created this campaign so that I don’t need to spend so much time when I start DMing a Planescape campaign (which means I can DM when I don’t have a lot of free time). I’m not sure if this is what people call a “sandbox campaign”, but whatever… in the end I have enough material to improvise upon (no struggle to create NPCs from scratch on the fly, or a new place, or even preparing statblocks for a combat encounter).
As for system, I have prepared stuff for D&D3.5 (that I will never, ever DM again), D&D4E (it is fun, though not the best fit for Planescape), Mage the Ascension (there is a pretty great adaptation, and fits better – but has many flaws as a system), Fate Core (can’t wait to try Planescape in Fate – I made a still untested adaptation: https://dl.dropboxusercontent.com/u/29427158/Fate%20for%20Planescape%20Alpha%200.1.pdf) and 13th Age (another cool system I want to DM). Most of the material is neutral, though, with some basic system-based stuff (mostly statblocks) that might be useful to improvise during a game.