Economics joins the ranks of those fields with an online academic journal. All papers are free for download. They've even back-posted all the old papers back to 1970. Feel free to click through and browse around. There's no registration or charge.
The next best thing to open access is preprint archives, the most prominent of which is ArXiv (pronounced "archive"). ArXiv is infrastructure to upload papers that are usually also submitted to a journal or conference. I first heard of preprint archives as used among physics researchers. Physics is a natural enough field to kick this off considering that they built the the World Wide Web to host their papers. Physicists publish in journals that have long review and publication delays, on the order of 6-12 months, and they seem to have realized that a 6-12 month ping time is not good for a group conversation.
I applaud BPEA going open access, and I wish computer science proceedings would do the same thing. Currently, all the American conferences are published through the ACM digital library, which has a dizzying array of subscription plans that has been designed to maximize profit. The current model in computer science is that CS research is IP for the ACM to sell for profit, much like a CD or a DVD. I would prefer a model where CS research is meant to advance science. Pay walls have a damping effect on discussion, and science without discussion isn't really science at all.