http://www.acuityathome.com - This site is used in public schools across Indiana. Students take Acuity Tests beginning in 3rd grade. They are used to predict whether a student will pass the ISTEP test. After students have taken one of the tests, they are assigned lessons based on concepts that they missed on the test. Students can easily access these lessons at home in order to practice those missing concepts.
http://www.scootpad.com - This site is wonderful! It allows students to practice their math and reading skills. You also have the ability to create additional assignments (spelling, writing, response to reading, etc). This is a site that costs money, but it has been worth it for me as a teacher.
http://www.mobymax.com - This site also provides practice for skills in reading and math. It also provides practice for vocabulary, math fact practice, and test prep! You can get this site for free or pay a fee for additional features.
http://www.raz-kids.com - This site allows students to read stories and answer questions in order to build their comprehension skills.
http://www.readtheory.org - This site allow students to read passages and answer questions in order to build their comprehension skills.