s6-portable-utils is a set of tiny general Unix utilities, often performing well-known tasks such as cut and grep, but optimized for simplicity and small size. They were designed for embedded systems and other constrained environments, but work everywhere. Other sets of small utilities are usually system-specific; for instance, the (otherwise excellent) BusyBox project only works on Linux.
s6-networking is a collection of small Unix tools designed to help networking. It includes clock synchronization, Unix and TCP super-servers, Unix and TCP access control, and other miscellaneous utilities. It is particularly suited for management of clients and servers on embedded devices, but works just as well on larger systems.
s6-dns is a small, efficient, complete, and IPv6-ready DNS client library, with synchronous as well as asynchronous APIs. It is designed to replace libresolv in projects that need to perform DNS resolution. It also comes with small command-line utilities to make common and not so common DNS queries.