Compared with Yomitan, a couple quick differences that come to mind:

- Manabi tracks the words and kanji you've read to show you which are new to you, and which you have as flashcards. You can see this visually on the page, and in a vocab listing

- Review flashcards that appear in whatever you're trying to read. Soon I will also have it auto-review flashcards passively as you read and encounter them naturally

- Add flashcards to Manabi Flashcards or to Anki including AnkiMobile on iOS

- One-tap words to look up instead of mouseover from starting boundary

- Manabi packages reading tools such as RSS, EPUB and soon manga (via Mokuro) with user-editable curated libraries of content. Yomitan is less of a standalone-capable tool

I am working on adding Yomitan dictionaries now (to also make the app multilingual) as well as more integrations such as 2-way sync with Anki, WaniKani, JPDB

I think Jidoujisho has a lot of similarities but it's not an iOS/macOS app

I should put up some product comparison material as there are a lot of tools out there