Chrome/Firefox/Safari: Grammarly's web app provides an easy way to proofread your text for writing errors, but the full features require a pro subscription. The free Grammarly Lite browser extensions, however, are available to everyone for instant spelling and grammar checks, as well as dictionary and thesaurus lookups.
Once you have the extension installed, just start typing, and you'll see a little Grammarly icon in the text field if the app has any corrections for you. Unlike other browser spellcheckers, Grammarly also points out punctuation mistakes, and you can double-click on any word to see synonyms or look up word definitions.
In my tests, Grammarly didn't catch some errors or know some words that After the Deadline caught, but the dictionary and thesaurus features were still handy. Even if you're not typing in a text form, you can look up words with the extension installed.
Some pro features require a subscription, such as checking for subject-verb agreement and word choice, but the free version still has your back for casual online proofreading.