GUI64 – GUI for the Commodore 64 running on top of the built-in ROM
c64-wiki.comToo bad it's not open source.
("NC" creative commons)
Related, Contiki OS is good inspiration and a really cool: https://csdb.dk/release/?id=8197&show=summary
Open Source and available on github. The old web site seems to be down which is a bit of a shame.
How much memory will be free for apps ?
One word: GEOS
GEOS comes with its own ROM and bypasses the Commodore 64 "Kernal". This one is a GUI that runs on existing ROM.
I used GEOS unironically in high school. I had two 1541 floppy drives which really cut down on the disk swapping and I was able to do productive work. Ultimately I used my employee discount to upgrade to a Tandy 1000 TL that would boot into DeskMate from ROM and took it to college, leaving the C64 in the dust. I still have all the stuff but not the time.
ContikiOS!
Limitations:
GUI64 does not use the VIC-II’s bitmap graphics mode, but operates in text mode. GUI64 is therefore character set–based... Strictly speaking, GUI64 should therefore be considered not a GUI, but a TUI (Text User Interface).
Currently, there are no applications for GUI64.
I never ran GEOS. I saw it advertised, and it seemed fascinating, but perhaps "too early". I had become fluent in BASIC and proficient at the command line. I could decipher the DOS commands and device IDs. For me, time spent on the Commodore 64 was programming, or in a game or application, rather than pushing files/objects around on the desktop.