One type I neglected to mention is "micro" (miRNA), and this is the one about which the journal Nature has a special on-line issue. miRNA, like other types in RNAi, binds to messenger RNA in cells to prevent gene translation. The special issue of Nature focuses on miRNA in various diseases involving tumors and skeletal abnormalities, and so far as I can tell, it's completely free to all!
What really caught my eye about this issue is its highly interactive medium, produced by some company called zmags. This "zmag" (I guess you'd call it?) has been rendered so that you view and leaf through actual magazine-like pages in your browser. I've got a 1+ yr old Macbook and the 2-finger zoom on the trackpad also works within the browser. Want to read and mark up some of it in your preferred program? Well you can save selected pages from the issue as a pdf, giving you flexibility in what content you download (though I did have some issues with this). A while ago I noticed Nature also used a somewhat interactive in-browser, pdf-viewing app called Readcube, though I admit I haven't really toyed with that program.
On this topic, Carl Zimmer had a neat piece in Nature a few weeks ago about the "rise of the e-book." He raises some excellent points regarding the pros and cons of e-books, some which I think could be extended to digital media more generally. I for one am like millions of others, relying on my handy computer and the internet for nearly all information I need to be a fully-functioning student, teacher and member of society. Still, as Zimmer points out at the end of his article, the permanence of e-books and the like is uncertain. I mean, what to do if we're hit by another devastating Y2k?
Nature special issue here
Zimmer, C. (2011). Technology: Rise of the e-book Nature, 480 (7378), 451-452 DOI: 10.1038/480451a