The Raspberry Pi 1 is still a great device to experiment with alternative operating systems, most of which are far less resource hungry than Linux:

- Plan 9 (http://9p.io/sources/contrib/miller/)

- Inferno (http://lynxline.com/projects/labs-portintg-inferno-os-to-ras...)

- RISC OS (https://www.riscosopen.org/content/downloads/raspberry-pi)

- NetBSD (https://wiki.netbsd.org/ports/evbarm/raspberry_pi/)

- FreeBSD (https://wiki.freebsd.org/arm/Raspberry%20Pi)

- Interim Lisp OS (http://interim-os.com - this runs on Raspi 2 only, so porting to the ARM v6 in the Raspi 1 would be a nice project) - btw., this is a project by Lukas Hartmann, who is also the creator of the open MNT Reform ARM laptop (https://mntre.com)

- (shameless plug) my bare metal "crosstalk" Smalltalk-80 (https://github.com/michaelengel/crosstalk)

I'm pretty sure this list isn't complete...

Some operating systems are not supported at the moment:

- OpenBSD only seems to support the Aarch64-based models 3 and 4

- Haiku seems to be looking for a maintainer for the Raspberry port