Command+P print a document from within a program
Command+S save a document from within a program
Command+O open a document from within a program. This will also open a selected icon or application from the Finder
Command+C copy highlighted text
Command+V paste copied text
Command+X cut text. Cut means to remove selected text, but copy it to the clipboard for pasting elsewhere.
Command+Delete Send a file to the trash from the finder or from a program like iPhoto
Command+Shift+N from the finder will create a new folder
Command+Shift+L create an alias of the selected icon
Command+I Get Info. This will give information about a selected icon or document
Command+F This is for Find. In a browser this will search a webpage. In the finder this will find a word in a file. This varies from document to document.
Command+G In a browser, if you enter a word you are looking for using this combo will look for the next occurrence of the word