If you have more than one key and use them for different purposes, you don't need to memorise the key fingerprints in order to tell them apart. PuTTYgen allows you to enter a comment for your key, which will be displayed whenever PuTTY or Pageant asks you for the passphrase.
The default comment format, if you don't specify one, contains the key type and the date of generation, such as rsa-key-20011212
. Another commonly used approach is to use your name and the name of the computer the key will be used on, such as simon@simons-pc
.
To alter the key comment, just type your comment text into the ‘Key comment’ box before saving the private key. If you want to change the comment later, you can load the private key back into PuTTYgen, change the comment, and save it again.