Recently I've heard a bit about the implementation (or rather, use of) /dev/null in Mongrel2, as well as other projects. However, I've never seen it explained what this actually means.
What does this mean, and why is it good for scalability (as I've seen it be claimed)?
Please read this :-) The Mongrel2 "support" was a joke (see the change, which was later removed).
Being serious though, /dev/null is useful when you want to discard output from processes. You can redirect output to it, for example, and the kernel will just discard that output.
/dev/null is a virtual device on UNIX systems which basically drops all data sent to it.