Open Access(OA) refers to freely available online information. Open Access literature is free of charge and often carries less restrictive copyright and licensing barriers than traditionally published works, for both users and the authors.
Essential Reading Open access overview by Peter Suber
Open Access to research increases its visibility, reaching a wider audience and therefore increasing its potential for:
Open Access Scholarly Information Sourcebook An authoritative source with practical steps for implementing OA
Open Access Directory Wiki with simple factual lists about OA