I use an app called Ella for a few minutes a day here and there, and I think it really helps to keep conjugations fresh in my mind. There are tons of conjugation apps, and I only tried 2 others (sorry can't remember their names). One of them was much more configurable and powerful than Ella, but it was hideous. All of the apps I tried required payment to unlock tenses, and you really needed to pay for any of them to be usable beyond basics. Ella is expensive at $22/year, but it doesn't have ads. The others were cheaper but had ads even in the paid versions.
What I like. #4, 5 and 6 apply to all the conjugation apps I looked at and aren't special to Ella.
Simple layout, easy to use interface
Includes keys for accented characters which makes typing easy. You have to type in the answers with correct spelling, which apparently some people do not like, but I do!
You can create your own personalized list of verbs to test
Can configure moods / tenses / pronouns to test. For example you can test first person indicative present or all subjunctive mood without vosotros.
Can choose whether the infinitive form of the verb is shown or if you have to translate it from English
1000 verbs with pre-made categories such as common, regular, irregular, reflexive, -ar, -er, -ir verbs
I also have a few nitpicks. The irregular category of verbs includes verbs that are irregular in any form, so there's no way to test verbs that are irregular in a specific tense / pronoun combination. The stats are bit weird. I wish I could add my own verbs (I'm looking at you, cocer), but I also understand that would be a big ask programmatically. And it would be nice if I could create more than one list of verbs.
I've read that Conjugato is a another great conjugation app, but from what I could see, the difference between Conjugato and Ella is whether you type in the answer. I don't think Conjugato has an interface for typing answers, and Ella requires it. I prefer typing because it reinforces spelling and accentuation, but I could also see someone preferring not to.
Overall, I feel like it really helps. I still have trouble conjugating on the fly in oral speech, but I've improved a lot.
I like it - it's a really good augment to the anki since I have to type out the verbs and I have to do a little each day to keep all the tenses above 90%.