I happened to see a mention of Stackby while I was searching for an online database service. Stackby is this weird love child of a database and a spreadsheet. It's neither of them, but somehow it's better than either of them. It's a great place to rapid prototyping, and with the API, it's actually a good solution for storing and manipulating data for apps.
There's some quirkiness in how Stackby works that takes some getting used to. They have mostly adopted the language of spreadsheets, so it feels very familiar, but there are things here and there that they decided to do differently, so you have to figure out their version of those formulas/commands.
REVISION: I am revising this part at the request of Rachit. As was pointed out, there have indeed been updates to the product, but as an example of the sort of "different" ways that Stackby does things, the updates page has the most recent updates at the BOTTOM of the page. So, unless you scroll all the way down, you don't see the recent updates, like I didn't.