Another project worth mentioning is [GlassOS](
http://www.cemetech.net/forum/viewtopic.php?t=5686&highlight...) which is an operating system for the TI-8x series calculators written in C and compiled with sdcc. It doesn't offer multitasking (or at least not background tasks, suspending and resuming might be planned, I don't know). It also supports some fairly gimmicky things like system-wide grayscale settings. It also hasn't seen any development for several months, which makes me sad.
KnightOS seems to be more focused on just providing a sane environment for user programs instead of TI's stock OS which is calculator-first and user programs are second class citizens.
I'm not sure how practical preemptive scheduling with background tasks is for these devices, they really are resource constrained. I'm sure it works fine with one background process, especially if it clocks up the cpu for that, but I don't see it scaling much beyond that. I'll have to try it out some time.
I'd be interested to see if a compatibility layer for programs written in C could be set up. SDCC's binaries tend to be reasonably performant, but significantly larger than their assembly equivalent. With 31k available to programs however, C could work for a lot of applications. I might hack on that sometime.