This is an idea piece.
Spreadsheet is a wonderful program for handling data. Now a day, almost all spreadsheet programs allows multiple sheets. Spreadsheet program usually offered facilities to allow user to see various part of a spreadsheet simultaneously.
One thing I haven't seen is to allow user to see different sheet of the same spreadsheet file at the same time. As we all know, data can be processed and passed/linked through various sheets and it is very likely that one would like to look at the original data to see if the data processing process is doing the right thing. The idea is, hence, to allow user to see various sheets side by side.
This idea can be extended to other kind of programs. For example, browsers today are becoming a dominant viewer. Again, almost all browsers provide the tabbing ability. But not many of them allow user to view tabs side by side, even though, most of the time, you can open a tab in a different window.