I am surprised by how many Photoshop junkies don't know this one:
While using the Paint Brush / Eraser / Clone Stamp tools, you can paint or erase a straight line by clicking once (don't keep the mouse button down) where you want the line to start, then shift-click where you want the line to end. Photoshop will paint a straight line between the two points. If you keep the shift key down, you can continue to click and add more line segments.
This can really speed up image editing.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050106193035936