Chemistry by Design
This site has over 1000 total syntheses arranged in note card format. Students can simply view the sequences are quiz themselves using a feature that removes reagents or products. I use it to pull real examples of transformations they see in the textbook. I have spoken with the creator and they are currently trying to get support to have a reaction class feature (ie SN1, SN2, Friedel-Craft, etc..) so you can search for published examples that way. It's excellent!