Horizontal mouse scrolling is supported for the Magic Mouse and the Magic Trackpad.

Unfortunately, not all applications which support vertical scrolling do support horizontal scrolling. The reason is that horizontal mouse scrolling was introduced "late" to Windows with Windows Vista.

There are many well-known applications which support horizontal mouse scrolling like:

Microsoft Excel does not support horizontal mouse scrolling (only Microsoft knows why).
As a workaround, use the middle mouse button to pan through your spreadsheet.