Freelance writing is a quick and highly liquid way to earn marginal income. During college and summer vacations I wrote for TextBroker[1], which pays per-word for SEO keyword articles in pretty niche topics. If you score well on their grammar and writing tests, you can make 1.5 cents per word or $7.50 per 500 word essay. The upside is that it's really easy to convert spare time into extra income -- you can claim one article at a time and write as many as you want. The downside is you'll be writing for some pretty scuzzy SEO folks, and if they ask for revisions it'll tank your cents-per-word estimate. It was a great way to round up some extra grocery money when expenses got tight, but it would have been difficult to sustain for many hours per day. On a good run I could make $40-50 over five hours before burning out.
I've recently come across Scripted, a more writer-friendly service that focuses on content quality instead of brute-forcing SEO keywords.
TextBroker: https://www.textbroker.com/
Scripted: http://scripted.com/