I heard of a great web site on Edmodo this morning. The website is Newsela. This has informational text and current events in several different subjects. This web site is free, has leveled reading passages, and built in quizzes (for some of the articles).
Teachers need to register for this site, but they can then create a classroom for their students to join. The terms of service state that you must be 18 to create your own account, but teachers can create accounts for students under 18 with parental permission.
On several articles there is an anchor with a number beside it. This is the ELA Common Core Anchor Standards. When you choose an article to read (teachers who create a class can assign articles to students; read about it in the FAQ section), on the right hand side is a blue bar. This allows you to differentiate between Lexile Levels for your students. Not all articles have quizzes attached to them, but those that do give you the grade level and standard that it meets. These articles would be good for grades 3 - 12.
image created by Dorene Bates at wordle.net