I am continually surprised at the new things that I learn about my camera. Despite having read much of the manual and used my Canon EOS 60D for more than 18 months, there are still a few tricks that I keep coming across.
I was taking some indoor photos in low light yesterday. The autofocus was not locking because of the low light and lack of contrast in the composition, so I switched to manual focus. Out of habit, I half-depressed the shutter button while I was turning the focus dial. Low and behold, one of the focus points in the viewfinder light up and the camera beeped as I adjusted the focus manually.
I went to the manual to find out what was going on, and sure enough, if you depress the shutter halfway while in manual focus mode, the autofocus sensor will alert you when you have achieved focus on one of the AF points.
This is a handy feature for me, because I normally wear glasses, and I sometimes find it hard to see accurate focus using the viewfinder. That small amount of assistance from the AF system could prove very helpful!