Now that I've been homeschooling for nine years, I have a good idea as to which supplies my daughter and I need to stock up on in order to make it through the year without having to go back and forth to the store.
Index cards: Index cards are a must. We use them for so many subjects. First, I have my daughter write her spelling and vocabulary words out on index cards each week to make studying them much easier. Next, we use index cards for gathering research for essays and school papers. The index cards also come in handy to create an outline on for an oral book report. When my daughter was younger, we used the cards to make multiplication flash cards.
Poster board: I usually grab at least 10 sheets of poster board for the year. We use one to create a time line for history, another to create collages based on what we are learning in science. Of course, there are always other projects that pop up throughout the year. Last year we did a project on the Civil War. The poster boards come in handy for art class as well.