Do you mean automatically, or only saves when you ask it to? Do you mean plain text, or formatted as the original HTML?automatically and I mean plain text so that it an be searchable
I've used and like both Surfulater
and Web Research
. Though neither of them will save your web pages automatically, all you have to do in either case is highlight the page, right-click, and select "Save article in Surfulater" or "Save article in Web Research." Both have excellent full-text search mechanisms. In Web Research, you can indicate that you want to save the article without graphics. There's no such option in Surfulater, though if you highlight just the text, you'll get just the text. It's also very easy to get rid of whatever graphics you don't want. Both programs let you arrange articles into categories/folders/whatever and use keywords, etc., but even without any of that, their full-text searches will probably locate very quickly whatever you're looking for among the pages you've saved.
I should also note that these are shareware, not freeware. They're both excellent programs. At the moment, I have a slight preference for Surfulater, which I think is somewhat more flexible and more fun to use, though more expensive than Web Research.