Dig Mandarin has about eight different types of courses, many of which focus on the HSK exams. You can specify which aspect of the HSK you want to work on, or you can take a full HSK course for each level that focuses on character writing, vocabulary, grammar, and HSK mock exams.
One of the teachers on the site uses animations and colourful videos for her lessons, while the rest use simple PowerPoint presentations. Watching their videos feels similar to attending a university class, which may or may not be your style. They do seem to thoroughly cover the HSK curriculum if that is your only goal — if your goal is to become functional in Chinese, however, perhaps consider investing a little more into courses from YoYo Chinese or ChineseFor.Us, especially if you’re a beginner to intermediate learner.
Besides the courses, Dig Chinese has some helpful articles, including some that differentiate between words with similar meanings (like 连忙, 急忙, and 匆忙). They also have sections on learning vocabulary, but these pages are simply lists of vocabulary words with an image and a recording — if you are determined to actually learn new words, use Pleco‘s Flashcards or Ninchanese instead.