Three levels of paragraphs for students to correct.At the advanced level to paragraphs seem on the short side Less than 10 questions per page. http://www.englishforeveryone.org/Topics/Paragraph-Correction.htm
http://www.grammar-monster.com/lessons/which_that_who.htm (especially useful: "Mistakes that will make you look really stupid")
Grammar Bytes: http://chompchomp.com/menu.htm interactive exercises, handouts, video presentations. This site looks great.