Just posted this in another thread but thought it was relevant here...90% of what was in the Call of Cthulhu book is available in other books.
* The insanity rules are available here:
http://www.d20srd.org/srd/variant/campaigns/sanity.htm
* The spells were all adapted to the Book of Vile Darkness almost verbatim, sans stat drains.
* DNDChick's page has conversions of all the monsters:
http://members.aol.com/CountryGrrlHere/cofcd20.html. DNDChick didn't agree with most of how Cthulhu monsters were statted up. After using them for some time in my D&D campaign, I have to agree with her -- use DNDChick's stuff, it's better than what's in the book.
* That leaves the Cthulhu gods. Try this older page for their statistics:
http://www.angelfire.com/extreme/kengage/cthulhu/cthulhu.html
* Weapons, armor, equipment you can get from D20 Modern:
http://www.wizards.com/default.asp?x=d20/article/msrd