MenuetOS

MenuetOS is a PC OS written entirely in 32bit assembly language and released under the GPL. Despite being almost entirely based in 32 bit x86 assembly, it might be a nice way to revive older hardware (as well as a bit more flexible than Contiki):

Features:

• Pre-emptive multitasking, multithreading, ring-3 protection
• Responsive GUI with resolutions up to 1280x1024, 16 million colours
• IDE: Editor/Macro Assembler for building kernel and applications
• TCP/IP stack with Loopback, PPP & Ethernet drivers
• Network applications include FTP/HTTP/MP3/SMTP servers, IRC, HTTP, NNTP and TFTP clients
• Free-form, skinnable application windows
• Hard real-time data fetch
• Fits uncompressed on a single floppy

Resources:

• Hardware Compatibility Page