You can also rotate on other axes. However, these operate a little differently. The 2 and the 8 key rotate your view with respect to the view from which you start. The steps are as follows:
Press the 3 key on the NumPad to get the Right view.
Press the 2 key on the NumPad several times to rotate positively around the Y axis.
Press the 8 key on the NumPad several times to rotate negatively around the Y axis.
Press the 1 key on the NumPad to get the Front view.
Press the 2 key on the NumPad several times times to rotate positively around the X axis.
Press the 8 key on the NumPad several times to rotate negatively around the X axis.