It would be remiss of me to not mention
Grammarly, which looks over work for common grammar and spelling mistakes including punctuation usage. It does require an account, but it's free (with extra, paid features of you're into that).
There is also
The Grammarist, a blog about phrase and word usage. They have entries on a huge portion of commonly mistaken words, as well as other fun stuff with word choice and idioms.
If you are writing across the pond and you're trying for the regional flavor (or flavour), you might want the services of
Oxford Dictionaries, which lets you search in UK or NA dictionaries (among others). It also has articles on grammar and different terms from American to British English.
I already mentioned
Draft, but what I didn't mention is that it has an option to add a word quota per day, with an option to email you reminders. It also tracks how much you write and during which hours you're most productive. (I recommend trying Draft.)