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Numbers

Numbers is a relatively new spreadsheet application from Apple, bundled as part of their iWork software. Numbers really shines in the user interface, and quality of charts and graphics produced.
It comes with a variety of different templates for various requirements, usually providing a good base on which to build. It also plays nicely with Microsoft Excel documents for good measure.
Price: $79.00
Developer: Apple
Requires: Mac OS X v10.4.11 or later

Microsoft Excel

The old workhorse. Microsoft Excel is available for Mac as part of their Office 2008 package, though comes at a fairly hefty price. The latest version has a much improved interface, though my personal experience is fairly dire. I find Office to be fairly slow, prone to crashing, and fairly poor at integrating with OS X in general.
If you need full compatibility with the .xls file format, this is certainly the way to go. Otherwise I would recommend considering the other options available before settling straight away for Excel.
Price: $149.95
Developer: Microsoft
Requires: Mac OS X 10.4.9 or later

OpenOffice

An impressive free spreadsheet utility is included with OpenOffice – Calc. It’s fairly intuitive and easy to learn, though has many of the advanced features expected of an Excel competitor.
One particularly useful feature is that of “natural language formulas”, which help to make writing a long formula somewhat less daunting. As with Numbers, Calc is capable of reading from and writing to the Excel file format.
Price: Free (Open Source)
Developer: OpenOffice (Open Source)
Requires: Mac OS X 10.4 or later

Tables

Tables is a fairly basic spreadsheet app, albeit with a very simple and clean interface. It has a wide range of calculation options, along with different settings to present data as a number, an amount, a percentage or as date & time (it covers the basics).
It is capable of producing some good looking charts, but doesn’t come close to those created by Numbers. Tables is a good mid-range option if you don’t want to shell out the extra $25 for Apple’s iWork suite.
Price: $52
Developer: Impressum
Requires: Mac OS X 10.4 or newer

Mesa

Mesa is a slightly dated spreadsheet application, but is still worth noting. It has multiple worksheets (like Excel) and uses a subset of Excel’s commands and functions (as well as some all it’s own). You can also produce charts and reports for easy export.
It doesn’t include macros or ODBC capabilities and lacks functionality found in the more expensive software.
Price: $34
Developer: P & L Software
Requires: Mac OS X Tiger (various versions available)