I love WaniKani. It takes a very specific problem, the need for English speakers to memorize kanji as part of learning Japanese, and it makes an entire product out of it. Sure, you can do the same thing yourself using Anki, but they add in a bunch of convenience layers. They think out memorable and ridiculous mnemonics for each character and each bit of vocabulary. They organize it as a big DAG of dependencies for what you learn that's loosely based around the order you'd learn it in school, but adjusted for adults. They put it all behind a really nice website that understands that most of their users just have an American keyboard and want to write things with English letters. There's a big community, regular "you're doing it!" positive update emails, and more. Really great example of taking one incredibly specific problem and just knocking it out of the park.
If I had one complaint, it's that people do frequently fall off the wagon, and once you do, Wanikani can make getting back on the wagon really intimidating. "Oh, you've been gone for three weeks? No problem, no problem, welcome back, let's se....you have 2,473 terms to review today." I recognize that you don't HAVE to do all of those thousands of things right away, but some sort of "I'd like to ramp back up slowly, please make the large scary number go away" button would be really, really great.