When you find something online that is useful to your genealogy research -- don't assume it will remain there to return to and review at a later date. If it's downloadable, download it and save it to your hard drive. If not, copy and paste it into a word document.
This may be easy to remember with personal, educational, or other amateur websites. But don't forget even the big websites can be forced to take things down by lawsuits, etc.
Google has faced some recent lawsuits over their Google Books project. I'm not positive it's related, but I have discovered that several books that used to be "Full View" (and downloadable in PDF form) are no longer available except in "Snippet View" at best, and in some cases no preview at all.
These are all books published prior to 1920, most prior to 1900, so it's not an issue of whether or not they were in the public domain. I'm not entirely certain what the issue was.