Open Access is a term used to represent both an idea, that the results of publicly funded research be freely available, and a movement, characterized by various initiatives to make published scholarly literature freely available on the web.

Quick overview videos:

Open Access 101  (from SPARC)
SPARC video blog

What is Open Access? (from PHD Comics)

Peter Suber, Director of the Harvard Open Access Project, Faculty Fellow at the Harvard Berkman Center, and Research Professor of Philosophy at Earlham College, provides a Very Brief Introduction to Open Access:

Open-access (OA) literature is digital, online, free of charge, and free of most copyright and licensing restrictions. What makes it possible is the internet and the consent of the author or copyright-holder.

In most fields, scholarly journals do not pay authors, who can therefore consent to OA without losing revenue. In this respect scholars and scientists are very differently situated from most musicians and movie-makers, and controversies about OA to music and movies do not carry over to research literature.

OA is entirely compatible with peer review, and all the major OA initiatives for scientific and scholarly literature insist on its importance. Just as authors of journal articles donate their labor, so do most journal editors and referees participating in peer review.

OA literature is not free to produce, even if it is less expensive to produce than conventionally published literature. The question is not whether scholarly literature can be made costless, but whether there are better ways to pay the bills than by charging readers and creating access barriers. Business models for paying the bills depend on how OA is delivered.

There are two primary vehicles for delivering OA to research articles: OA journals and OA archives or repositories.

• OA archives or repositories do not perform peer review, but simply make their contents freely available to the world. They may contain unrefereed preprints, refereed postprints, or both. Archives may belong to institutions, such as universities and laboratories, or disciplines, such as physics and economics. Authors may archive their preprints without anyone else's permission, and a majority of journals already permit authors to archive their postprints. When archives comply with the metadata harvesting protocol of the Open Archives Initiative, then they are interoperable and users can find their contents without knowing which archives exist, where they are located, or what they contain. There is now open-source software for building and maintaining OAI-compliant archives and worldwide momentum for using it.
• OA journals perform peer review and then make the approved contents freely available to the world. Their expenses consist of peer review, manuscript preparation, and server space. OA journals pay their bills very much the way broadcast television and radio stations do: those with an interest in disseminating the content pay the production costs upfront so that access can be free of charge for everyone with the right equipment. Sometimes this means that journals have a subsidy from the hosting university or professional society. Sometimes it means that journals charge a processing fee on accepted articles, to be paid by the author or the author's sponsor (employer, funding agency). OA journals that charge processing fees usually waive them in cases of economic hardship. OA journals with institutional subsidies tend to charge no processing fees. OA journals can get by on lower subsidies or fees if they have income from other publications, advertising, priced add-ons, or auxiliary services. Some institutions and consortia arrange fee discounts. Some OA publishers waive the fee for all researchers affiliated with institutions that have purchased an annual membership. There's a lot of room for creativity in finding ways to pay the costs of a peer-reviewed OA journal, and we're far from having exhausted our cleverness and imagination.

An excellent profile of Open Access is available in the Transforming Scholarly Publishing through Open Access: A Bibliography

Below are some examples of open access initiatives.

Public Library of Science
The Public Library of Science, one of the pioneers in open access publishing models, is a non-profit organization of scientists and physicians committed to making the world's scientific and medical literature freely available to the public.

Berlin Declaration on Open Access to knowledge in the sciences and humanities
The Berlin Declaration on Open Access to Knowledge in the Sciences and Humanities was signed in October 2003 as an outcome of an international conference on open access.

NIH Public Access Policy
Stipulates public access to NIH-funded health related research information by means of articles being deposited in the PubMed (National Library of Medicine journal article database).

Budapest Open Access Initiative
The Budapest OAI arose from a small but lively meeting convened in Budapest by the Open Society Institute (OSI) on December 1-2, 2001

SPARC Open Access Newsletter
A monthly newsletter tracking developments in the open access movement

Open Access Tracking Project
For those staying informed up to the minute - daily updates via blog