www.4teachers.org is the home of 15 FREE tools that teachers at every grade level can use! Here you'll find RubiStar, QuizStar, NoteStar, and a number of others that are highly intuitive and extremely practical for day-to-day classroom work. If I had to select one "favorite" site, this would probably be it -- it offers so many tools from a single portal.
If you're not familiar with Kathy Schrock's Guide for Educator's, it's worth 10 minutes of your time to explore. Each week, Schrock scours the web and serves up the best of the best via her web site and updates to your inbox. Follow this link to Schrock's "Site of the Day" master page. Sign up to receive new sites via email.
Forget the old "make a poster" assignment and let students log onto Glogster to create multimedia, interactive online posters. Students can collaborate on group posters via the site, and Glogster offers a wealth of ideas for teachers to use in the classroom. FREE for the basic tool.
Visual Thesaurus is an AMAZING interactive vocabular tool that provides students with a tech-savvy, visually intriguing web interface. Years ago, this online tool was free for educator use. Unfortunately, there is a charge for site use now, but you can still try it out free for 14 days.