I'm a freelance editor for Edanz Editing. Most of my work with them is editing manuscripts that were written by non-native English speakers in order to help get them published in English-language journals. I also do some other random tasks, like helping authors select journals for publication, preparing cover letters to be submitted with manuscripts, and recommending peer reviewers for publications. Edanz requires a background in academia or a PhD-level education, but the pay is pretty decent once you become efficient at editing. As an extra perk, I enjoy having the opportunity to read articles on a variety of topics ahead of publication and the work is interesting.
I also do a good bit of freelance writing in my field (veterinary medicine) and some in the human medical field. Most of my work in that area has come through Upwork.com (and previously Elance.com), with a mix of short-term and ongoing projects. Right now, most of my energy is going to an ongoing project with a company that advertised in the one of the veterinary journals. I currently make about $2-3k month in about 10 hrs/wk.... and that is with no active efforts on my part to seek out work, aside from the one application to the job advertised in the vet journal. (I haven't pplied for a job on Upwork in 6-8 months, but periodically I get "invitations to interview" that are worth looking into.)
I look forward to the day when I can give up my FT day job and go exclusively freelance, but my husband isn't going to be comfortable with that idea anytime soon. For now, I'll keep my FT job for the benefits, the freelance work for the mental stimulation and extra cash, and continue my quest for a FT home-based medical writer position!