Oh why oh why did I have to discover longform.org? I will never get any work done! I haven't looked into the Instapaper feature on the site, but here's the gist of what longform offers: a central clearinghouse of long-form narrative past and present. Like "The Radioactive Boy Scout," a story I loved but would never have remembered where I read it (Harper's, 1998).
I found longform by way of a Poynter's piece, "How Technology Is Renewing Attention to Long-form Journalism."