multitos.. it had its strong points.. but nowadays there is freemint (multitos follow-up). it's a really flexible OS and runs at decent speed (as opposed to multitos).
you can try multitos, it's easy to install and you'll get a taster of the multitasking capabilities. but after a while freemint is a more suitable (but harder to configure) system. you can use decent filesystems and xaaes is a really smooth aes (gui layer) replacement.
