Spelling and grammar checking features are often part of word processing software
MS Word (spelling and grammar check)
Google Docs (spell check only)
Contextual spelling and grammar checking apps
"Check it" feature on the Read & Write for Google Chrome toolbar (requires paid license)
watch demo video of this feature
Grammarly.com (free or pay for premium features)
Google Docs not currently supported. Use online Grammarly Editor, then copy/paste into Google Doc.
Ginger software (free or pay for premium features)
LanguageTool (free GoogleDocs add-on) or LanguageTool (website)
Ghotit software ($99 Android/iOS app; $199 Windows/Mac)
ProWritingAid is a a grammar checker and style editor