Remember, the library has copies of reference books, too...and it's often just as easy and quicker to find what you need in a book than online.
Almanacs - yearly books filled with facts on almost everything! They are especially good if you are looking for statistics like the length of a river, the population of a country, sports scores, etc.
Dictionaries - books to look up what a word means, how to spell it, say it, and more.
Thesaurus - lists words that are close in meaning, or synonyms; often list words that are opposite in meaning, or antonyms. They are helpful when you don't want to use the same word over and over.
Atlases - books of maps and geographical information
National Geographic MapMachine - search this online atlas of street maps and satellite images. See if you can find your house in a satelite image!
Encyclopedias - books to look up general information on all branches of knowlege. They are helpful to use first when starting research on a topic.
World Book Online - you will need the school's username and password; ask your teacher or teacher-librarian for it