Toybox combines many common Linux command line utilities together into a single BSD-licensed executable. It's simple, small, fast, and reasonably standards-compliant (POSIX-2008 and LSB 4.1).
Toybox's main goal is to make Android self-hosting by improving Android's command line utilities so it can build an installable Android Open Source Project image entirely from source under a stock Android system. After a talk at the 2013 Embedded Linux Conference explaining this plan (outline, video), Google merged toybox into AOSP and began shipping toybox in Android Marshmallow in 2015.
Toybox aims to provide one quarter of a theoretical "minimal native development environment", which is the simplest Linux system capable of rebuilding itself from source code and then building Linux From Scratch and the Android Open Source Project under the result. In theory, this should only require four packages: 1) a set of posix-ish command line utilities, 2) a compiler, 3) a C library, and 4) a kernel. This provides a reproducible and auditable base system, which with the addition of a few convienciences (vi, top, shell command line history...) can provide a usable interactive experience rather than just a headless build server.