The announcement is a few days old, but I missed it (and it doesn't seem to have turned up on the lists yet), so:
http://about.jstor.org/participate-jstor/individuals/early-journal-content
"On September 6, 2011, we announced that we are making journal content in JSTOR published prior to 1923 in the United States and prior to 1870 elsewhere freely available to anyone, anywhere in the world. This “Early Journal Content” includes discourse and scholarship in the arts and humanities, economics and politics, and in mathematics and other sciences. It includes nearly 500,000 articles from more than 200 journals. This represents 6% of the content on JSTOR."
http://about.jstor.org/participate-jstor/individuals/early-journal-content-f...
Access is through the normal JSTOR interface (which can, if you wish, be tweaked to only display open content). It's not currently all available, but is being rolled out in chunks.