Rotating tiles

press "R"

Or "V"

Here's a complete rundown:

H - flip a tile horizontally
V - flip a tile vertically (yes, the same button as to paste, so make sure you don't have anyting in the clipboard)
R - Rotate a tile 90 degrees clockwise
Shift+R - Rotate a tile 90 degrees counterclockwise

BTW, it used to be that you had to press N after H or V to 'normalize' the tile and make the flip permanant. This is no longer the case.