Middle School Resources
Ardor Math - Personalized or teacher led math practice. There is the option for a free account for up to 41 students, or other pay options.
Edcite - Offers many courses (all middle school math courses are there), where teachers create a classroom and add students and assign units of work. It is a little daunting to search through their activities and lessons, but I saw some good ones for middle school students.
Global Math Stories - These are individual activities that are based on various countries and geographies. The would be great supplemental material when you (and the students) are tired of working analytical problems.
Help Teaching - Lots of math self paced lessons for all grades. Not a complete curriculum but the lessons contain videos and problems sets and seem well put together.
i-Ready (From Curriculum Associates) - I looked at some of their free math packets and I liked the problems and completeness of the packets. Since they are all free they might be good to download and keep for future years. Make sure you check out the addition resources as well - there are a lot more materials here.
Illustrative Mathematics - Many of you may have heard of Illustrative Mathematics (this began as open-up resources but were adopted by Illustrative Math). Many teachers really like the materials, although I have heard they need to be supplemented with additional practice problems. This may be the math textbook of the future so you could get a head start on the materials now!
Long+Live+Math at Home - Blend of textbook and online resources. Here is a spreadsheet with links to all of their materials, there are tabs for all grades at the bottom. This is a great resources organized by topic.
Khan Academy - There are Khan Academy Courses taught by "Real" math teachers that include practice problems and assessments. I think the quality of Khan Academy has come a long way. Here are the Links for Middle School