This isn’t really an advanced spell checker, but I actually kind of like using spell. Especially if you want to maintain your own dictionary to add jargon and slang. That being said, I don’t spellcheck all that often so maybe if you use it a lot it becomes inconvenient (?). Also, I know if you use vim you can :set spell

I use open source Language Tool (https://github.com/languagetool-org/languagetool) within my not-open-source sublime and it works great.

You can probably find a foss way to use it:

LanguageTool’s core functionality is open source. Anyone can set up their own LanguageTool server locally or in the cloud. Furthermore, we are looking for contributions to further improve our proofreading technology.