I haven't touched the trial subscriptions to blogs, magazines and newspapers at the kindlestore, yet. But I'm about 1/4 of the way through the http://tinyurl.com/gutmagic list of titles hotlinked to *.mobi (kindle readable) formated Project Gutenberg books (including audio books there - many more than I expected; also many foreign language audio & text versions of the books). I browsed the 2,000+ non-fiction titles offered free (always or temporarily) in the kindlestore itself and either downloaded or added to my wishlist for future download.
The nice thing about getting kindlestore titles, free or paid editions, is that the full support comes with it: book highlights, bookmarks, notes will be accessible online at one's kindle account, for example. Other sources I have in mind to glean and load onto the kindle include poems at The Writer's Almanac, the transcript sections of some podcasts, and the transcript sections that accompany PBS content. The simple way to do this (unless already usable in PDF, presented for download "as is") is to use Evernote.com to accumulate selected passages of text and then export all those evernote entries as a single HTML document that can in turn be sent via email with the Amazon convert service.