Click and drag the upper portion of any window toward the top left of your monitor screen. When it hits a certain sweet spot, it will suddenly fill exactly one half of the screen on the left. Take a second window and throw it to the right (by clicking and dragging the upper portion of that window towards the top right of your monitor screen). You should now have a perfectly split screen with the two windows you are working on.
This is great when you are creating a PowerPoint from a Word document or adding some figures from Excel into a report in Word.