For my money there is nothing to beat Encyclopedia Astronautica, maintained by enthusiast Mark(?) Wade. It isn't the prettiest site on the web, but the information content is both superb and surprising.
It deals with every aspect of astronautics, containing a lot of information from the Russian side of the fence that is otherwise difficult to obtain, and is well supplied with data on lunar exploration.
Encyclopedia Astronautica
I've always been fascinated by the various plans to use a Gemini adaptation for the first lunar landing attempt. This could (if it had worked) have put an American on the moon a year earlier than Apollo.
By Gemini to the Moon!
