Google Chrome, like most other browsers out there, comes with a built-in spell checker. The feature is actually more advanced than the one in other browsers as it relies on Google's existing spell-checking technology, the same one that powers the "Did you mean" feature in Google Search.
Even so, the spell checker isn't perfect, no such tool is. The ability to add custom dictionary words is great in this case.
With the latest Chrome builds, in the dev channel now, it's easier to see changes to this custom dictionary, as well as add new words or edit existing entries.
To do this, you can go to the settings page and search for "dictionary" or go directly to chrome://settings/editDictionary.
What's more, it seems that Google is working on making this custom dictionary syncable, meaning it will stay with you no matter where you use Chrome. Obviously, all of this is useful if you rely on the built-in spell checker, many people don't.