Open Astrophysics Bookshelf (also on GitHub) – “A collection of open-licensed ‘living’ textbooks on topics in astrophysics” – maintained by Michael Zingale
Firefly, an interactive web-based particle viewer by Aaron Geller and Alex Gurvich
Multiwavelength Astronomy from the University of Chicago’s eCUIP Digital Library collection (Note: These resources were developed for high school classrooms rather than college classrooms.)