italki is one of the most useful websites for learning any language, Korean included. On their platform, you can book private classes directly from a tutor. It's much cheaper than you may expect, often less than $10 per hour. Additionally, they have lots of useful extra features, such as language exchanges, and their notebooks section where you can get free feedback on your writing. Review.
Talk To Me In Korean (TTMIK) is one of the most often recommended resources available. This site has a ton of content for those learning Korean, some of which are free and others require purchasing. The material is available for everyone from beginner to advanced speakers. You can find books, video lessons, audio lessons, tips for learning Korean and a bunch more. TTMIK is definitely worth checking out.
Pimsleur is one of the most well-known language learning courses around. In the past, I had a hard time recommending them because they were too expensive. But, they've recently added a subscription option which makes things much more affordable. Compared with other courses, Pimsleur is much better at getting you to speak and actively use the Korean you've learned. However, this comes at the cost of clear grammar explanations. Review.
Naver is the most used search engine in Korea. They also have an excellent online dictionary which lets you look up words in Korean or English. More advanced learners may appreciate the Korean-Korean dictionary as well. Additionally, you'll find lots of example sentences, audio recordings, along with their word of the day, and more.
Lingodeer is a lot like Duolingo, except much better for Asian languages. Their app is free to use and includes tons of lessons and exercises for you to practice what you've learned. Their wide variety of exercise types helps keep things interesting and challenges you to learn all aspects of the language. They also have in-depth grammar explanations and game elements to help keep you motivated. Review.
Of course, you won't master Korean in 90 days, but 90 Day Korean's Module 1 course could give you a really solid foundation. They've since received a lot of positive feedback and added in new modules for those who have already progressed past the beginner level. In addition, a Korean teacher will check in with you via email to help keep you accountable.
KoreanClass101 is an excellent resource for finding lots of high-quality audio lessons. The content available ranges from the beginner to advanced levels. Lessons typically focus on a dialogue spoken very naturally. Throughout the lessons, there are lots of explanations to make sure you've understood the content. Beginner level lessons include lots of English which is gradually phased out as you move up levels.
Memrise is right up there with Duolingo among the most popular language learning resources. It's not so great for teaching Korean as a whole but is quite good for teaching individual words. It basically uses flashcards that have been designed to be a bit more fun. While there are some courses created by Memrise, the majority are user added. Because of this, the quality and material taught can vary significantly depending on the course. Review.