This question already has an answer here:
What operating systems would you recommend for this type of hardware?
Though I keep the question text generic because answers can work for many others, but my specific case is a Laptop Siemens Nixdorf PCD-4ND 486 /DX4/75 (24Mb RAM, 420Mb HD, Res. max. 640x480)
The laptop has diskette drive and a ethernet card, so this are the way to install or move data in.
At the moment I have 2 harddrives, one with windows95 and the other one with a very old and outdated Debian Linux.
I would prefer OSes including graphical interface but also actively supported CLI OSes are welcome.
(I am sure that there are opensource/not main stream projects and distributions which are still supporting this old platform (i.e freeDOS), so if you are no aware of the scene please don't answer saying that there is nothing)