Here are some of my favorite sites for lesson planning inspiration and ideas. All the sites listed are for multiple grade levels, but do skew towards grades 6-12.

Math Assessment Project
Full lesson plans and materials for awesome rich tasks and conceptual lessons for all grade levels.

Mathalicious
Full lesson plans for real life lessons with media tied to common core standards. Easy to search but requires subscription – which is worth every penny!

Desmos
Powerful online graphing calculator and so much more! Lessons and activities are easy to find and implement with the whole class. If you have access computers for students – this is an awesome site!

Youcubed
Site full of rich tasks and information about fostering a more open math classroom. There are so many different resources on this site – you are bound to be inspired!