Reading
https://www.readworks.org/ (Reading Comprehension-high interest fiction and non-fiction passages with comprehension questions)
https://www.starfall.com/h/ (great for phonics and comprehension, there's a Math section too!)
Math
https://www.khanacademy.org/math/k-8-grades : (online practice in all areas of Math from K-8)
Writing
https://readwritethink.org/ (Grades K-12-reading, writing practice)
https://boomwriter.com/ (Grades 2-6 practice for all writing skills)
https://FunEnglishGames.com/ (Grades 2-6-writing in a game format)
https://StoryboardThat.com/ (Grades 2-6 interactive practice by creating a storyboard as a graphic organizer.
https://Storyjumper.com/ (Grades 2-6 Interactive resource where writing is broken down into steps on a graphic organizer to be put together as a whole story)