Gmail's keyboard shortcut for composing new messages is "c", but there are two undocumented shortcuts that let you compose a message, while also adding the CC (carbon copy) and the BCC (blind carbon copy) fields:
* b - compose new message and add both CC and BCC
* d - compose new message and add CC
To use these shortcuts, you first need to enable keyboard shortcuts in Gmail's settings page. Gmail is the only Google service that doesn't enable keyboard shortcuts by default.