There are a lot of really neat pages out there that deal, in some way, with the Hellenic Underworld. Some may just give you another opinion on this information, some may add information for your own collection, and some may be super scholarly or super religious. I leave judgement on them for you.
The Temple of Hecate Yes, this is religious. There is a really neat fortune telling game, and some beautiful pictures, but not a huge amount of textual information on Hecate. Still, this is a gorgeous page.
The Pythagorean Pagan A modern day revival of Orphism-Pythagoreanism (religions based on literature and math, neat!). This page has information on the religion as well as a ton of Orphic hymns.
Hellenic Paganism Modern day information on the worship of Apollo, Dionysus, Hades, and Persephone.
The River Styx A fan fiction and art gallery for the Hades from Disney's Hercules. What could be worse?
Rethinking the Halls of Hades A neat article on a place that might have inspired Homer's Halls of Hades in The Odyssey.
The Xenaverse Codex We're getting desperate, aren't we? Actually, it is rather intersting to see how Hercules and Xena portrayed various Gods. (Hades was not a blonde though...I stand by that one.) There are some very intersting Hades fan fiction stories, though unfortuantely (or perhaps fortunately) Persephone doesn't appear very often.
The Feast of the Assumption Have you ever heard that Chritianity and Orphism are a lot a like? This is what happens when fanatics get ahold of Persephone....
Hades' Walk-in Closet In this very silly page you can hide in a closet with Persephone and Adonis and learn about intersting things to do in the Underworld.
Osiris and Persephone Oh? And you thought that it couldn't get worse than Sailormoon, Hercules, and Hercules?
Regional Folklore and Mythology A really neat grouping of tons of differnet myth and legend pages on many different topics. (Mine is or at least was the only Hellenic Underworld one, though)
Lake of Dreams Though the theme is poetry, a large portion of the poems are mythological. (Besides, it's my page. It has to be good, right?)
Ivory Tower
This is another of my pages, with a "favorite characters of Julia" theme,
which includes this page, the writing page, and a ton of other mythical
and legendary characters, some from Greek Myth.