SheetJS Live Demo
SheetJS Data Preview Live Demo
(Base64 text works back to IE6; drag and drop works back to IE10)

Source Code Repo
Issues?  Something look weird?  Click here and report an issue
Output Format: 
Drop a spreadsheet file here to see sheet data
... or click here to select a file
Advanced Demo Options: Use Web Workers: (when available) Use readAsBinaryString: (when available)