TLS Books is a free site with a lot of great and high quality printable worksheets and activities for all grade levels and subjects.
Education.com is free, but you have to become a member to download the worksheets. There are a lot of other resources on the site as well including educational activities for each grade level.
Super Teacher Worsheets is another great free resources full of printable worksheets for all grade levels.
Crayola is a great site for exploring art projects to do with the kiddos. It has everything from printable coloring pages to lesson plans. You may need to buy some art supplies, but the kids will have a blast.
Starfall and the Starfall Download Center have amazing learning games and videos as well as printable books and worksheets for your preschool-2nd graders, and it’s all free! They have new area called More Starfall which requires a paid membership. It includes K-2 Math, a Talking Library, and songs, rhymes and phonics. They also have iPhone/Ipod touch apps available.
I don't always print the worksheets. Sometimes they work great on the iPad! I grabbed a free printable fraction work sheet from super teacher worksheet. I opened the link on my iPad and saved it by pushing the power button and home button at the same time. Then I opened the picture in Adobe Ideas, a free app for drawing and photo editing. From there my son could fill in the answers and color in sections of the fraction. He had fun making the answer the same color as the colors in each shape. There may be other apps that you could do this same thing,this was just one I already had.