General Castlevania Links
Sites that have lots of info about numerous CV games, such as The Dungeon, Castlevania Quarters, etc.

Castlevania Game-Specific Sites
Sites that are specifically for one game; they may have small bits of content for more than one but the large majority is for one game.
Miscellaneous CV Related Sites
Great miscellaneous sites, such as sites for ROM hacks, artwork, sprites, scans, etc.
Castlevania Music Sites
mp3s, midis, sheet music, guitar tabs, you name it
Castlevania Fanfics
Sites specifically for one CV-related fan fiction
Konami Official Sites
Konami of America, Konami of Japan, Konami of Europe.
Useful for Castlevania news, release dates, and pictures.
Other Helpful Videogame Sites
Sites with any game info or codes (such as IGN, Gamespot, GameFAQS etc)
Translation Sites
Automatically translate those pesky CV sites that are in Japanese, Russian, Portuguese, etc

Additional Searches
Automatically search Google, Excite, About, etc. for tens of thousands of other CV-related sites not listed here.

Also be sure to check out The Castlevania Dungeon for it's game sections, music, and fanfics.