I use http://www.furl.net/ which satisfies the requirement mentioned: "a good solution would have to keep copies, not just references". Furl lets you save the text of the page you have visited, as well as the link. It saves them on the furl server, so you can furl from any machine. I notice that Furl has become less popular. I don't know whether people moved to http://www.spurl.net/ instead. I think the pdf solution may be best for the long term.