Here are some items that are handy when you go to write adventures:
Your Game Books! The PHB, DMG, Monster Manual, Etc. The campaign book is good if you’re using a prepackaged campaign, too. Also keep any sourcebooks you’re drawing from handy, too. This one’s a no-brainer.

A dictionary. It’s helpful in a lot of ways besides propping up the couch. You can look up long, obscure words that some game designers think make them sound more intelligent. It will also help when you write, not only with spelling, but it’s great for generating names, adventure hooks and locations.

A thesaurus. Many of the same reasons above plus it keeps you from sounding too redundant. Every DM can benefit from giving detailed descriptions. It’s also good to help separate NPC dialogues. Different characters use different words and phrases.

A foreign language dictionary. Especially Latin. The benefit here is that you can name spells characters and locations and make them sound older, more well-traveled and exotic. Latin is especially good for spells and old tomes. You can also substitute real life languages for foreign languages. “Look the gnomes speak Spanish! Hola amigos. Donde estan los hombres malos grandes?” Latin is also great for ancient languages like Draconic or Ancient Elven.

A Baby Name book, or if that’s too scary, the Phone Book: It’s great for making up names on the fly. Need something that sounds weird or fantastical? Try spelling in backwards or leaving a few consonants out. You can also radically alter a name with adding an X,Y or Z in place of the first letter. Warning!: Pfred the Dwarven Barbarian doesn’t sound all that tough. Make sure the name is appropriate to the character. : )

Some kind of style reference: This can be anything from the AP style guide to an old TSR module. If/when you find a format you like for your adventures, keep it on hand to guide you when you get stuck. It’s also good if you’re submitting your adventure to someone to have their guidelines on hand. Another benefit of having a more “professional” style guide on hand is to help with grammar and commonly misspelled words.

Another Game System, Campaign or Gaming Book. Sometimes inspiration comes from looking at another game. Sometimes it’s just easier to borrow a whole concept entirely. (When you borrow, don’t publish unless you’re willing to give the other guy some credit.)

Maps or an Atlas: Not just the game map, although that’s great, but keep a world or the map of the country you live in on hand. This is great for establishing distances and getting an idea of the scope of something. “It’s about like running into Mt Everest in the middle of the California Badlands...over here at starvation point…” A fantasy world set on the map of the real world could be interesting if you move some of the terrain around and add a few things.
Sorry, the numbers didn't copy... Doh!