Smashing Magazine turned me on to Handsontable, “a minimalistic approach to Excel-like table editor in HTML & jQuery.”

Handsontable features:

• Automatic adding of new rows and columns
• Undo and redo
• Copy & paste
• Column and row headers
• Scrollbars within containers
• Drag to fill cell values
• Right-click context menu
• Custom buttons
• Conditional formatting
• Prepopulating new rows from a template

Sounds very cool.