Here is a list of dictionary apps for the Mac. If you know of others, or if details need updating, please let me know.
MDBG — A Chinese–English dictionary that can be added to the built-in Mac OS Dictionary app. Free trial version includes (old) HSK level 1–4 words. A larger paid version is available for $19.90 (Mar. 2011).
Xiao cidian — Two dictionaries that can be added to the built-in Mac OS Dictionary app (Ch-Eng and Eng-Ch are separate). Free (Mar. 2011).
UsualDict — An extensible dictionary app. Includes a Chinese dictionary, but can be expanded with dictionary files in StarDict format. Note-taking feature. $4.99 at the (Mar. 2011).