Formulas aside, you can roll your own Excel clone fairly easily. Have a normal HTML table, capture the cell clicks. On a click, replace the cell's HTML with that of a text box containing the same text. Capture tab, shift-tab and enter keypresses and you're nearly there.
Well, it would need to be able to add rows/columns and delete them. Copy/pastability would help, too. A necessary pre-requisite for that is going to be the ability to select entire columns at a time -- that's going to have to be custom-done because your browser isn't that smart. Let's see, an undo wouldn't hurt. Oh, and the escape out of editing mode. And resizing when you overflow a single cell. Speaking of which, drag and drop resizing on arbitrary rows/columns.
There's probably a few requirements I'm missing, but no problem, I can whip up a Stack Overflow clone in like a weekend and solicit suggestions on what one or two trivial details I still need to implement to duplicate the thousands of man years in Excel.
I'm not sure whether it wasn't obvious enough, or you're being a tad disingenuous. I was talking about a very simple implementation, which has worked for me on a number of occasions.
Incidentally, if you weren't just nitpicking and really do rely on copy/paste between Excel and a web-based spreadsheet you need to be careful. JavaScript only has access to the plain-text clipboard from Excel. This is just the stuff you can see. If anything has been truncated for display purposes in Excel, it'll be missing from the copy buffer.
