Sheetsu is the simplest backend for startups from Google Spreadsheets and hosting from Google Drive
Zoho Sheet is a cloud-based spreadsheet application that allows real-time collaboration, while providing all of the essential spreadsheet features. The Zoho Sheet mobile app for iOS and Android will make it much easier to work on the go.
TotalSheets is a platform that offers you all the tools you need to create a fully functional online spreadsheet. Similar to Google Sheets in concept, but with the intention to give a greater control to owners over the appearance of their published spreadsheets.