I'm just finishing designing a campaign with a viking / Norse theme. Feel free to email me to start talking ideas; it'd clog bandwidth to do it here. My address is jatfairfield -at- yahoo -dot- com.
Sources
That said, here are some good sources:
David Lawson's new book The Raven (available free in its entirety at
http://www.tenthousandmonkeys.com/monkeys/compline.htm) is as awesome as it gets; the entire last half of the book has a great Norse theme. And, you can see the D&D worldbuilding underlay under the whole thing.
Thirteenth Warrior has been mentioned already.
Avalanche Press has a book called Viking Age out that includes the material from the Doom of Odin and other great books of theirs. The new classes and powers are intentionally WAY overpowered -- I don't use them -- but the cultural terminology and look at Viking life is very useful.
Frostburn has lots of new material that's adaptable to a Norse setting; so does Races of Stone.
Class Adaptations
As far as class adaptations, there's a lot in the core books that's easy to transpose to a viking context. I adapted Hammer of Moradin to be the baseline warrior of Donar (Thor) class; Frenzied Berzerkers as the leading warriors of Woden (Odin); and Shieldmaidens of Sif (Champions of Gwynharwyf) as the leading female warrior adaptation.
Other classes are easy to adapt, as well: Runecasters, Runesmiths, and Battlesmiths already fit.
I used Mystic Theurge as my primary Priest of Woden class -- the emphasis on knowledge, versatility, plus the idea that Woden alone among the gods mastered the (otherwise feminine) magical arts of seithr means that the only "ok" magic-users are also Wodenic priests.
Arcane magic is otherwise limited to female wizards ("Glythja"), female sorcerers ("Spaekona", who are outcasts and midwives), foreigners (who are looked down upon for their "feminine diversions"), and dwarves.
Races and Monster types:
Speaking of Dwarves (or Dwarrow), I changed their favored class to Wizard, and left them otherwise alone.
Fey, I've developed as a major evil and the primary enemy race. I called them "Svartalfr", and they're soul-sucking, child-stealing, evil-on-a-stick just waiting to happen. Dryads are not your momma's tree-hugger, and nymphs will suck your soul out to warm their freezing hands, returning you to your village as Soulless, a new subtype (if a Fey "kills" you via energy drain, immediately change alignment to Chaotic Evil, all compunctions gone, intense desire for murder and other unpleasantness, PCs become NPCs under GM control).
Undead also were transformed. They're no longer inherently evil -- in fact, many undead ("Draugr" -- for regular undead; "Hagbui" for mound-dwellers) are the result of tragic actions in life, and exist to seek people out who will right great wrongs.
Trolls are another major faction. The Monster Manual III has many new variants of Trolls, and I plan on using them a lot. The major issue is that I've made them subject to extreme sunlight sensitivity (turn to stone if caught in true daylight) but given them extra magical powers in return.
I renamed Goblin Forestkith as Bush Trolls -- weak enough for characters to begin encountering Trolls from first level.
There are lots of other ideas, but I think this post is long enough; I like to keep the crunch-to-noise ratio fairly high.
best,
Carpe