PubMedCentral has a new feature: a search can now be limited to Open Access Articles! Very cool! The irony of creating Limiting, to find the Unlimited, is simply delicious!
Check this out at at Entrez PubMed PMC. Click Limits.
Total PMC open access articles as of May 13, 2008: 70,588.
Free and open access are obviously not the same thing in PMC terminology; the total number of free research / review articles is 1,139,216.
Fortunately, PMC posts its Definition of Open Access, based on the Bethesda Statement on Open Access Publishing:
An Open Access Publication1 is one that meets the following two conditions:
* The author(s) and copyright holder(s) grant(s) to all users a free, irrevocable, worldwide, perpetual right of access to, and a license to copy, use, distribute, transmit and display the work publicly and to make and distribute derivative works, in any digital medium for any responsible purpose, subject to proper attribution of authorship, as well as the right to make small numbers of printed copies for their personal use.
* A complete version of the work and all supplemental materials, including a copy of the permission as stated above, in a suitable standard electronic format is deposited immediately upon initial publication in at least one online repository that is supported by an academic institution, scholarly society, government agency, or other well-established organization that seeks to enable open access, unrestricted distribution, interoperability, and long-term archiving (for the biomedical sciences, PubMed Central is such a repository).
Comment: kudos to PMC for posting their definition of open access! It would be great if all search services did this.