While doing some editing checks recently, I found the Purdue Online Writing Lab - https://owl.english.purdue.edu/owl/ and have used it several times since then. It has been helpful, and I am sure I will use it often in the future.
It is well organized and easy to navigate. The site has a good search feature and several categorical listings on the side of the main page. After I choose a topic, rules of grammar pertinent to that topic pop up, and there are several examples of the rule in sentences, with both incorrect and correct usage/corrections. This feature was extremely helpful to me in the lesson on dangling modifiers and how to correct them. It helped clarify a few points in my mind through the examples and explanation.
These features make navigating and searching through the site quick and efficient. If I cannot find the item using the search feature, I can use the site map to help navigate.
In addition to helps for grammar, the site also features sections on pertinent topics such as: Undergraduate Applications; Research paper helps (with both APA and MLA style information); Online Tutors; a Blog with questions, answers and examples; types of writing (i.e. creative, technical, literal, social science, engineering, etc); there is even a section with ESL topics.
This site has been helpful to me this semester, I wish I had found it earlier so I could have used it in my other classes. It is on my bookmark toolbar now!
Now for your editing enjoyment, take a look at a newspaper clipping my brother sent over earlier this week. While it isn't an error in grammar, it shows the importance of proof reading.