The mouse-cursor is the single most watched icon for any computer users without a doubt, but did you ever cared to think why it is tilted and not straight? Well somebody did and asked about it at "User Experience" (UX) section of Stack Exchange - the popular Q&A website and finally got the answer.
Well the answer is:
The mouse, and therefore the mouse cursor, was invented by Douglas Englebart, and was initially an arrow pointing up.
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 recognizable than the straight cursor.