KnitPicker
KnitPicker calculates stitch counts for rows based on short-hand instructions.
It works with Mac OS X 10.3 and up, only.
To use KnitPicker, download the archive, then double click it in the Finder. Then double click on the KnitPicker application, and you will be able to enter the short-hand notation (ie. k4, p2, k2tog, etc.) You can then see the stitch count automatically updated.
You can download it here or the source.
Comments, feature requests, etc., should be emailed to James.