For AWS I think some of the best offerings on their free tier are Lambda and DynamoDB. Both of which have generous lifetime free tiers. Using the two of them you can have an API and database that is good enough for a lot of startups and hobby projects without paying a dime (ever). Together they are used for Amazon Alexa skills which means hosting your Amazon Alexa skill is free for most skills.
I think DynamoDB is waning in it's already limlited popularity. It has some clever ideas, but is really hard to implement. The main selling point is price, but you kinda get what you pay for.
It's actually pretty easy to use too if your use case makes sense for it.
However, if your use case does not fit what DynamoDB expects, you are better off using another DB rather than try to bend DynamoDB to your will.