KeyRocket is an interesting piece of software that works outside of Excel (aka not an add-in), and suggests Excel keyboard shortcuts based on what Excel options/features you use the most.
This program can help if you are constantly forgetting shortcuts as it allows for pop-up notifications reminding you that there’s a shortcut you could have used.
It is free for non-commercial use and also works with Word, PowerPoint, Outlook, and Windows Explorer. Update: Per the comments, the software is no longer free for non-commercial use.
Download here if interested: KeyRocket – Excel Keyboard Shortcuts
Especially useful for beginners to help get familiar with keyboard shortcuts early!