Radius made for the Macintosh in the long, long ago.
They're not in the biz anymore, but you can find plenty of pivoting monitors sold by the major brands. Some of my coworkers really like having one monitor in landscape (for spreadsheets, coding, etc.) and one in portrait (for documentation, web pages, etc). If you want one, do your research: portrait mode may not support wide viewing angles well, and font rendering may be screwy (because sub-pixel rendering assumes horizontal sub-pixels, not vertical ones). Also, unlike your smartphone, pivoting monitors don't necessarily contain a sensor to automatically detect changes in orientation: you have to tell the OS to display output for you, then physically rotate the display.