Have you ever wondered why your mouse cursor rests ever so slightly to the left? Chances are, that little arrow on an incline is so ubiquitous that you've never even thought twice about its 45 degree lean. As it turns out, there's a very good reason for it. Or was, anyway, back in a more pixelated age.
This is the historical reason:
(concept drawing taken from document: http://bitsavers.trailing-edge.com/pdf/xerox/parc...)
When the XEROX PARC machine was built, the cursor changed into a tilted arrow. It was found that, given the low resolution of the screens in those days, drawing a straight line and a line in the 45 degrees angle was easier to do and more recognisable than the straight cursor.
As you can see below, the original, straight cursor was indeed much more difficult to pick out amongst the blocks of basic text.
As Gijssens points out in a later edit, after Englebart created the left-leaning cursor, Steve Jobs borrowed it for his software followed by Bill Gates who borrowed it after him. At this point, we've just become so accustomed to our leaning (and still highly functional!) arrow that anything else would just seem too bizarre. Besides, why mess with perfection? [Stack Exchange via Digg]