Developer Citadel5 packs GS-Calc full of interesting features, including a large work area (4094 columns by 12 million rows), pivot tables, robust charting, and a nice interface for dealing with multiple worksheets in a single project.
GS-Calc has an unusual, and welcome, take on multi-page spreadsheets.
Rather than a row of tabs along the bottom to handle multiple sheets, which becomes problematic when a workbook grows beyond four or five pages, GS-Calc offers a hierarchical, folder-based view supporting multiple levels of folders. This makes it easier to create a workbook consisting of many smaller, more-focused sheets, a boon to navigation and debugging.
GS-Calc also offers an interesting method of generating multiple values.
A single formula can fill several cells with calculated values. These "array formulas" work with most of GS-Calc's functions. For example, entering "=SQRT(4)+{1;2;3;4;5}" will produce the numbers 3 through 7, in five cells, beginning with the one in which the formula was entered. If the data in one of the "generated" cells is overwritten or changed, it is refreshed as soon as the worksheet recalculates itself.