Free World U has the primary goal of creating an online learning experience for all students in grades pre-K through twelve. One of the key features of Free World U is the large collection of flashcards. Free World U categorizes the flashcards by grade level and subject area. A lot of flashcard services claim to be comprehensive, but Free World U really does have a comprehensive collection. For example, when I clicked through the high school collection I then could select eleventh grade followed by US History. From there the US History category offered a large selection of sub-categories of US History.

Applications for Education
Free World U offers a great selection of review materials appropriate for any grade and content area.

Here is a related item that may be of interest to you:
10 Places to Make and Find Flashcards Online