Is there an xkeymacs equivalent in Mac? In PC, the utility of xkeymacs.exe enables one to use emacs key in every case. Does Mac have the similar utility? For example, is there a utility that I can use key 'C-x C-s' to send the key 'Command-S'?
You could configure some of them in ~/Library/Keybindings/DefaultKeyBinding.dict. See hcs.harvard.edu/~jrus/Site/cocoa-text.html and this sample file: Emacs Opt Bindings.dict.

Another option would be to use KeyRemap4MacBook. It supports over 1000 preconfigured keybindings, including an "Emacs Mode" section.