The Unofficial Definitive Guide to the DC Universe has an index series, a chronology, Who's Who, and a history of the DCU.
The DC Comics Database Project is a wiki dedicated to the DCU.
Supermanica: The Encyclopedia of Supermanic Biography is pretty self-explanatory.
The Great Book of Oa has a complete bios of (almost) everyone who has ever worn a power ring.
Flash: Those Who Ride The Lightning profiles all the members of the Allen and West legacies, as well as their friends and foes.

MV1 Fanfiction is where is all started so long ago.  There are tons of regular titles there!
DC Anthology Fellow DC fanficcers who have been at this nearly as long as we have!
DC Infinity They call themselves the next step in the evolution of DC Comics fanfiction, and who are we to argue with that?
The Five Earths Project What if the original, separate continuities of Earth-1, Earth-2, Earth-S, Earth-4 and Earth-X had continued beyond the Crisis on Infinite Earths? This site answers the question with all-new stories set across the old multiverse, including Earth-3, Earth-C, and more!
The DC Treasury Sadly not all fanfic sites live forever. But some of the fondly remembered ones are gathered here in archive form, including All-Star 2000, DCHeroes, DCU Dark Side, and DC:Year 2.