Sheetsu is the simplest backend for startups from Google Spreadsheets and hosting from Google Drive
Calcapp is an app designer for spreadsheet users with support for almost 300 formula functions. Lay out your interface visually, connect formulas to fields and share your app with co-workers and customers. Add multi-level menus to help your users navigate, buttons that e-mail PDF reports and validation rules that flag bad values. Apps work offline.
Algo by AirDev
Turn a Google Sheet model into a web app in 5 minutes
How Algo works:
1) Sign in with Google and connect to the spreadsheet that you want to use.
2) Choose what cells you want to send to the model and what cells you want to receive from the model.
3) Use your app to enter values, run them through the model, and get the results. Make your app public to share with others.