Search by image
These are listed roughly in order of usefulness, although it's worth being aware of each as they can have different strengths depending on the task at hand. A good way of having access to multiple reverse image tools in one go is to use a browser extension; from my own use, I tend to prefer Image Search Options in Firefox and RevEye when using Chrome.
Powerful service. Tends to do well on facial recognition relative to other services
Very strong. Comprehensive and has useful filters
Lags a little behind the big two currently, but has some interesting features, particularly the ability to crop to elements of an image and re-search in real time
Making something of a comeback after lagging for a few years. Strong on logos and symbols, and can filter chronologically
Automatically removes the background in an image. This can help reverse search engines as other objects in the image being searched can cause confusion