Some tripods and tripod heads incorporate spirit levels to help you get the camera level. Spirit levels are especially important in panoramic photography, where any slight tilt is magnified across the width of the panorama. Here, the spirit level must be in the tripod base; it's essential to have a level platform for the tripod head. Spirit levels may be horizontal (more accurate, but one is needed for each axis of movement) or 'bubble' types (less accurate, but only one is needed).
Some cameras now incorporate 'digital' levels which are displayed on the LCD on the rear of the camera. These help prevent tilting horizons, but only work along one axis and aren't always as responsive or as accurate as conventional bubble levels.