WordReference is my favorite online dictionary because it provides various uses, colloquialisms and the ability to create or search for questions about vocabulary/phrases/idioms that native speakers answer. You can also add this as an app on your phone.
SpanishDict is great for studying grammar, verb conjugations, and pronunciation. You can take free practice quizzes there, too.
Study Spanish is also another great website for grammar and free quizzes. However, some of the quizzes require membership.
Spanish Cognates is great for easily acquiring more vocabulary to your repertoire.
50 Spanish Adjectives to learn and practice.
My YouTube playlist.