"Open Access is the free, immediate, online availability of research articles coupled with the rights to use these articles fully in the digital environment."(SPARC, the Scholarly Publishing and Academic Resources Coalition).
There are multiple ways for scholarly works to be open access:
Check this list for more disciplinary repositories:
Multidisciplinary open access journals
See the list of open access journals published by major publishers
Publishers can offer a mix of traditional and open access articles or they offer free delayed access to their publications. See the individual journal home pages and check what articles are available for free.
For any articles you cannot access (databases or Google) I would suggest using additional tools that could help finding open access versions of those articles (meaning pre-prints or post-prints). These tools are: