Protothreads are a type of extremely lightweight, stackless threads written in 100% portable C. While protothreads were originally designed for severely memory-constrained systems such as deeply embedded systems, they have found a number of uses as a general purpose portable library as well.
Contiki is an open source, highly portable, networked, multi-tasking operating system for the Internet of Things. Contiki includes a multitasking kernel, a TCP/IP stack and a set of application programs, and a low-power radio communication stack. It is written in C and designed to be very small: it runs comfortably in a few kilobytes of RAM.
uIP is probably the smallest implementation of the TCP/IP protocol stack ever written in a high level language; the code footprint is on the order of a few kilobytes and RAM usage is on the order of a few hundred bytes. uIP is intended to be used in embedded systems running low end 8- and 16-bit microcontrollers.
uVNC is a very small VNC server that can be run even on tiny 8-bit microcontrollers commonly found in small embedded devices. With uVNC, such devices can have a networked display without the need for any graphics hardware or a computer screen. A demo server running on a Commodore 64 is available.