The most widely used target for shooting is the X-Rite 24-patch ColorChecker Classic. The traditional one is 8×12 inches. There is also a pocket version called the X-Rite ColorChecker Passport Photo, which is useful when shooting on location in unusual light (such as an indoor arena or manufacturing plant.) These targets can be used for white balance, exposure confirmation and building custom camera profiles. There are a few others that are useful.
The AIC PhotoDocumentation Target (AIC PhD) is attached to a magnetic strip that can be included in a photo to provide a permanently embedded color reference. It is a common tool in museum photo departments, published by the American Institute for Conservation of Historic and Artistic Works
The IT8 Targets are typically used to calibrate scanners. As you can see, they have considerably more color patches than the X-Rite. IT8 targets come in reflective and transparency versions. They are a valuable tool if you are using scanner software that supports calibration with IT8. Software includes VueScan and SilverFast.