ReadTheory is a free and interactive reading comprehension program. Students begin the program with a ten question pretest that determines their baseline reading level for the program. Then they are given leveled passages to read and answer multiple choice questions on. When they get the question right, they get a knowledge point. When they get it wrong, they are shown the correct answer and an explanation of why that answer was the right one. Basically, it is an interactive version of a reading comprehension workbook. The site is free, but it is funded through ads so there are popups which can be distracting.