Notepad - Handbag or pocket-sized is ideal to carry with you. You never know when something amazing is going to pop into your head, or when you are going to overhear some random conversation that could be the seed of a great writing idea. If you don't scribble it down, you are likely to forget it. It's also handy to keep a notepad by your bed, as dreams can offer up unusual ideas - many writers have used dreams and nightmares as a inspiration for their novels.
If you are caught short without your trusty notepad, use the memo or note application on your mobile phone.
Pen - Your notepad is useless without it! I always carry a couple with me, as one always seems to stop working when I need it most.
Computer - As satisfying as long-hand writing is, it needs to be typed up at some point if you are going to submit it. Computers can be evil though; there is so much distraction with email, YouTube, Facebook, Twitter etc etc. Ignore these and open up Word then keep writing.
Of course the Internet is useful for research, but be careful where your sources come from - not everything you read is reliable.
Reference Books - These can be novels you adore; sometimes it helps to look at the format and style of other novels. This is no excuse to plagiarise though - just use as inspiration.
A dictionary and thesaurus are great tools, as are grammar books, atlases and the like. Don't get caught up with the details though - you can check on spelling and grammar when you have completed your first draft, otherwise you'll be languishing in editing hell after only one chapter (I should know).
Babies name books are great for character names. Just make sure the name fits the character; a popular cheerleader is unlikely to be called Morag, and an evil crone, Jessica. Sometimes the meaning of the name is a good influence. I also sometimes use the phone book or name searches on Google to find surnames for my characters.
Music - If you don't find music a distraction, use it. I like to make soundtracks for my novel. I visualise my plots in a very cinematic way, and imagine what music would be used if it ever made it to the big screen. Twilight author Stephanie Meyer popularised this idea by posting her soundtrack choices on her website - I got the idea in primary school when a teacher asked the class to write a story and choose music to go with it. It's easy to create your own playlist, especially if you can use iTunes.
Sometimes songs can be too distracting to me however. I can easily start singing along and find myself thinking of the lyrics instead of my novel. Instrumental music is helpful for when this happens; movie scores and nature cds are great for setting the mood. Last.fm has a movie score station, and for iPhone owners, there are nature 'sound' applications you can add. I often use the storm sounds.
Scrapbook - The same teacher I mentioned before, also asked my class to cut out pictures of people and places to represent the characters and settings in our stories. It's something that I still find useful to this day. I try not to use really famous people as templates for my characters - I find it difficult to imagine my protaganists as Tom Cruise or Jessica Alba anyway. Often I use several different people to represent one character; I just pick and choose the features I want. You can also make cuttings of cars, houses, pets, clothes - anything and everything; very handy for adding to your character biographies if you do them.
I also put recipes and news articles in my scrapbook. Anything really that inspires my story.
Tea - Okay, so Tea isn't as cool as her ubiquitous sister, Coffee, but she does the same job. There is still that caffeine hit to wake you up. I don't like coffee (though strangely I like the iced versions). Tea is my fuel of choice, and when nursing a cuppa (I like good old PG Tips with soya milk), it feels like a hug on the inside...awwwww.
Food - Make sure you have breakfast, lunch and dinner (snacks too if needed). Otherwise you'll find your mind wandering to the contents of the kitchen cupboards...
