Skritter is fun and engaging (if not downright addicting!). Its repetition-based system is very well scheduled.
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you can use numbers to add tone marks to pinyin. For example, if you type “Xi1an1,” it will be converted to “Xī’ān.”
lets you input Chinese characters, which it will then transfer into pinyin alongside the meaning of the characters.
The Pleco app serves as a dictionary for the Chinese learner allowing them to search by pinyin, or by radical.