I have listed many homeschool math problems over the last couple of years, and many of them have included middle school resources. At the cost of being redundant, I will list middle school math resources that may include programs I have listed before.
Khan Academy has elementary to middle school math videos where the math concepts are explained through video. I would use this program plus and middle school math workbooks for 6th grade math/ arithmetic (level 1-7), Pre-algebra (7th grade) level 8-11 , and Algebra 1 (8th grade) levels 12-13. If you use Khan Academy's Free adaptive math program, you will also find that Geometry is sprinkled in between the units... or you can go to the main page and follow video tutorials in the same order as the pages in your math workbook.
AAAmath.com lists math by grade level. Sixth grade math teaches place value through scientific notation and patterns. Seventh grade math teaches fractions, decimals, percents, and basic geometry. Eight grade math teaches more Geometry. You can also opt to teach algebra on from this site for 8th grade math.
Internet4classrooms.com has math lessons and resources arranged by grade.
It is important to remember that middle school math is basically review of all of the previous years at a greater level of difficulty, and then pre-algebra. If the student is ready, they will then do algebra. For my son who is very successful in math, he took two years to learn pre-algebra, then followed that by algebra. The review sixth grade math is not really necessary unless the child has struggled in previous years, but you will want to take your time with pre-algebra to ensure they have a good understanding of the concepts.