ABBA Village is the outstanding ABBA mailing list which also has all the latest ABBA news.
Carl Magnus Palm, the author of the most amazing ABBA books and most liner notes of the latest ABBA releases has a very informative site of his own.
An excellent website which list hundreds of ABBA tv appearences throughout their career is Sara Russell's ABBA on TV.
And of course you can go to the special Wikipedia pages about ABBA.
The website for the official ABBA endorsed exhibition, ABBAWorld promises an exciting and historic trip.
Also the official ABBA web site ABBA - The Site is worth a visit.