Study Champs is a free service offering online practice quizzes in mathematics, language arts, and science. The online quizzes are essentially online worksheets that offer instant feedback as to whether or not you've entered the correct answer on each question. Study Champs also offers printable worksheets.
Applications for Education
I'm generally not a fan of worksheets or fill-in-the-blank online activities, but I know that some people do like them. If you are looking for some simple online practice activities for your students to use after your lessons, Study Champs might have what you want.