While it is great having grading books, there are times that having a grading app on your Android or iPhone device just makes life easier. This is particularly true when you are at your local coin show and are doing a stare-and-compare with a coin you are thinking of buying.
One of the best out there is PCGS Photograde. It is an app available on the web, for Android, and for iPhone and best of all it is free. It gives you a guide for grading coins, ranging from About Good (AG) all the way up to Mint State (MS) 68. Each coin in each grade has high resolution images that you can zoom into and see very detailed features of each coin so you can compare it to your own coins you are grading or considering buying.
PCGS Photograde is setup by coin denominations such as Half Cent, Cent, Nickels, Dimes and so forth. If you dive into a particular coin, such as Cents, you will then get types of it such as Liberty Cap Large Cent, Coronet Large Cent, and of course Lincoln Cent. Tap the one you want to explore and it will bring up that coin with high-resolution images that you can swipe back and forth to see different grades. When you are looking at a particular grade, you can pinch and pull to zoom into that coin to get greater details.
One of the great things about PCGS Photograde is that it helps you not only do that stare-and-compare with your own coins, but it is also a great tool for learning how to grade coins too. Grading coins is as much of an art as it is a science and professional graders from PGCS, NGC and others spend years learning by looking at thousands of coins. Think of PCGS Photograde as a bit of a cheat sheet to help you learn and get better at it.