jmc

There are only two kinds of rectangles of area 8 square units which we can form on the grid: $2\times 4$ rectangles and $4\times 2$ rectangles. For $2\times 4$ rectangles, the upper left unit square of the rectangle must be in the left three columns and top five rows, giving 15 possible locations. Similarly, there are 15 possible locations for a $4\times 2$ rectangle. Therefore, the total number of rectangles of area 8 which can be formed is $\boxed{30}$.