All the 35 hexominoes, in a SVG document. They're coloured according to their symmetry groups:

20 hexominoes (coloured black) have no symmetry. Their symmetry groups consist only of the identity mapping
6 hexominoes (coloured red) have an axis of mirror symmetry aligned with the gridlines. Their symmetry groups have two elements, the identity and an orthogonal reflection.
2 hexominoes (coloured green) have an axis of mirror symmetry at 45° to the gridlines. Their symmetry groups have two elements, the identity and a diagonal reflection.
5 hexominoes (coloured blue) have point symmetry, also known as rotational symmetry of order 2. Their symmetry groups have has two elements, the identity and a 180° rotation.
2 hexominoes (coloured purple) have two axes of mirror symmetry, both aligned with the gridlines. Their symmetry groups have four elements.
Hexominos.png by AndrewKepert
Júlio Reis