Edhelper.com — I only wish the website was better presented (cleaner, better organized)
Edhelper is full of ready-made worksheets, mini-books, activities, whole units etc. for levels from pre-k to high school. Teachers love Edhelper because they can supplement their activities and create homework easily. The subscription is $20 a year and $40 a year for all levels, up to high school. It's pretty worthwhile. I only wish the website was better presented (cleaner, better organized).