Probably the most versatile free and open source (those 2 don't always go together even though a lot of people think that they do) is Inkcut. It is a python program that depends on Qt GUI widget. You can install it with PIP if you have Python installed on your systems. I run only Linux on bare metal and that comes with Python, Macs do come (or at least did) come with Python, but it was 2.7 (if I recall correctly), which has since been deprecated and no longer supported, don't know if they moved to Python 3.6+ or not (or removed Python entirely), Windows would need to install python and then get PIP.
If the machine is offline, at least on Windows, it is possible to get everything as offline installers and move it over manually without PIP. Possible for Linux as well as Linux already comes with Python installed, not to sure about Macs.
That's the only one that I am aware of that's free that does have quite a few options for automating tasks and going current with software/OS etc. They do have a listing of what machines that they support.
The other 2 options would be to see if you could cut directly from whatever design software that you have. The easiest way is if your plotter isn't so old that the vendor still has drivers out there and could possible use that and the "Print" function within your design software to cut directly to the plotter. Downside to this is that everything has to be manually done by the operator in order to work. Overcuts, weeding boxes, nesting etc, all manually done. If that doesn't work, a more manual work around for that, but still use the "Print" function of your design software would be to setup up the cutter as a "printer" manually and have the language as RAW HPGL to communicate between the two. Exceptionally easy to do (and do it via GUI) on the platform that I am on, but I didn't try this using the more mainstream OSs, so I can't comment as to how easy it is on those as I was no longer on those OSs.
The last option would be to VM(can get VM software for free, perhaps the most well known and robust is Virtualbox) the older OS(if you have the activation key and an install disk/ISO) and software on the newer computer. Since you are communicating with an older machine and using newer computer parts to host the VM, that does make communicating between the two more difficult. Certainly not impossible, but it will take some time to get it up and running. It'll be easier if the plotter relies on USB, but I have a feeling that it is serial and that does require more work.
Let me re-phrase this. So, your boss calls up and demands that you come to work for free now...
I can actually understand this since the OP said that they only "use it here or there". Depending on what "here or there" means, it could make for good fiscal sense to minimize the cost related to that hardware, without ditching the hardware totally. If overdo it there, cost has got to give somewhere else to make up for it.
Now there is a point of diminishing returns on older hardware, but I also think people are to willing to get new(er) when they don't necessarily have to. As long as it doesn't compromise what the business is able to do and how they are able to do it, and the machine isn't at the point to where it's just a money pit, keep it going.