Arxiv has a narrow focus (only physics) and it's rather difficult to even get permission to publish there in the first place. Cold Spring Harbor Laboratory runs BioRxiv and MedRxiv for biology and medicine, respectively, but again there's the issue of focus. Those are the big three. Then IEEE started TechRxiv as a preprint server for tech/engineering, which, given the landscape at the time, was a good move.
It's very easy to get access to arxiv.org, if you're at a university or a company doing research you'll surely have a colleague to vouch for you. Even if you're a lone wolf, just ask someone on a subject-focussed site or email a local professor.
