Much of the development I do these days uses vagrant machines to make sure that my code is running in the correct environment. This is great, but spinning up too many machines at once can rather stretch the resources of the computer you're running them on - and I keep starting up machines and then forgetting about them!
It turns out that (since vagrant 1.6) you can ask vagrant to tell you which of its machines are running, using the command:
Where did all my system resources go? Now I know!