From its home page (see below):

The Contiki desktop environment is a highly portable, modern, open source, Internet-enabled OS and desktop environment for very constrained systems, such as 8-bit homecomputers like the Commodore 64.

Internet-enabled means a web browser (in 9 kilobytes and 4 kilobytes of RAM required) and an e-mail client.

This OS is form Adam Dunkels, the same author of μIP.