Open Access Directory hosted by Simmons College: "a compendium of simple factual lists about open access (OA) to science and scholarship, maintained by the OA community at large."
Open Access Overview by Berkman Fellow Peter Suber at Harvard University
Why Open Access? from the Scholarly Publishing and Academic Resources Coalition (SPARC)
The Budapest Open Access Initiative (BOAI) contains the original text of the BOAI declaration, as well as resources in understanding the OAI.
Removing access barriers to this [scholarly] literature will accelerate research, enrich education, share the learning of the rich with the poor and the poor with the rich, make this literature as useful as it can be, and lay the foundation for uniting humanity in a common intellectual conversation and quest for knowledge.
Open access journals provide scholarly content freely and without restriction online. Different kinds of OA journal publication models offer varying degrees of access and use different business models.
Many journals offer all of their content freely. Others may offer hybrid access, wherein some articles are free while others carry a fee. In delayed open access journals, the most recent articles require payment, while older articles are free.