Flubaroo is a free script that you can use grade the quizzes that you administer through Google Docs. Flubaroo provides great step-by-step directions for using the script. I'll give an quick overview of how it works. First, create your multiple choice quiz using Forms in Google Docs (get directions here). Then take the quiz yourself and have students take the quiz (you can embed it in a webpage or direct students to the URL for your form). Now instead of trying to grade the spreadsheet cells you will insert the Flubaroo script by selecting it from the "insert" menu in your spreadsheet. Once the Flubaroo script is inserted just select it and it will grade the quiz for you.