There are two main options:
Gold Open Access Publishing
This is when the journal you publish in makes digital content freely available immediately upon publication.
This can be done in two ways:
Open access journals generally publish your work under a copyright licence, rather than an assignment of copyright.
This is the difference between you retaining copyright ownership of your work and the journal owning your publication.
Green Open Access Publishing
This is when you continue to publish in traditional subscription-based journals, however the publisher allows you to deposit (self-archive) a digital copy of the article manuscript online.
You will usually be allowed to deposit the author’s final manuscript in either a subject-based or institutional repository (ie. UC’s Research Repository), or by posting the article to your personal website.
Many authors prefer the green option because it allows you to continue publishing with well-established journals in your field.
There are many reputable publishers who produce open access journals including PLoS, Hindawi and BioMed Central. However the onset of open access publishing has also seen an increase in vanity publishers (those who charge an author fee but without peer review) and these can be difficult to distinguish from legitimate publishers. It is important to assess whether the publisher is reputable as well as the cost to publish.
Jeffrey Beall: Criteria for determining predatory open-access publishers (2012)
As a potential author, be wary of publishers who:
As a researcher, be wary of:
Having said this, not all of the above are necessarily signs of untrustworthy publishing practices. The hard sciences, in particular, have had a long tradition of rapid scholarly communications and in some disciplines multiple-authored research put out very rapidly under peer review is the norm. For instance, see Nature Communications.