Are there websites that maintain lists of "must have" English SF books?
As an example, in French, there is La Bibliothèque Idéale de l’Imaginaire from Le Cafard Cosmique, which loosely translates to The Ideal Library of the Imagination.
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Well these are these two websites. This site, The Classics of Science Fiction has got a pretty good list, based on a citations metric, i.e. the number of times they are mentioned in blogs, sites, email, across the web. Some books I've not read. Worth a look.
The second subsite, found 162 Classics of Science Fiction 1996 is also based on citations as well. The second list is much more detailed. The citations detailed, are if they have been mentioned in the reference works at the top of the article.
Another list can be found at Scifi Lists. There are actually 200 books listed as well as short stories, movies and TV shows.
Another good site is http://greatsfandf.com/, which despite its dated look was last updated June of 2012.
There was a list that was maintained since beginning of the nineties by Tristrom Cooke. Unfortunately, the list hasn't been maintained since 2003 (as far as I know). Still it's a great list. A copy can be found here.