Apart from my regular consultations with my all knowing oracle (Google) I have a few other things I refer to often when diving into the unknown depths of my literary world.
First and my most favorite, note cards. I use them by the hundreds. I write down facts and ideas but the most fun I have is outlining with them. I write down my scenes and lay them all out then I can shuffle them around for pacing purposes.
Tied for second place is my dictionary and thesaurus. I use the thesaurus more because I have a tendency to use the words "mammoth" and "frozen" a lot and need alternatives.
And coming in third is my Encyclopedia of Magical Creatures by John & Caitlin Matthews. Always looking to be a little creative with my paranormal creatures. Did you know there is a Mongolian Death worm named Olgoi-Khorkhol. It is about four feet in length, dark red in color and spits highly corrosive poison at its attackers. So if you are ever wandering the the Gobi Desert be carful. There is also fun and pretty things like fairies and dragons as well.
What things do you find useful when you are writing?