Litcharts is a wonderful, relatively new resource for teachers and students. It's more sophisticated and in-depth than Sparknotes, and it covers a range of texts and genres.
My high school students and I used it extensively last semester during our sonnet unit. For example, the analysis of Shakespeare's Sonnet 73 offers a line-by-line summary and a thematic discussion of themes and topics. Some of the content requires a paid subscription (they offer a school license), but much is free! I find the content to be invigorating and accessible; it's one of the best resources I've encountered recently, and I recommend it to all of the teachers with whom I work.
-Mike Macasulo
ICTE Past President
University of Notre Dame