j: In the stream, you can click ‘j’ to navigate down to the next item
k: Navigate up.
@ or +: mention someone in a post
q: While on the Home tab, press twice the q letter on the keyboard to search and add people to your chat list.
Space Bar: Scroll down stream.
Shift + Space Bar: Scroll up stream.
Tab: Scrolls through comments/users on a post
Enter: Hitting Enter when focused on a Post opens up the comment box.
Tab + Enter: End comment.
Here are few more shortkeys.
Being able to use shortcuts when posting or commenting is very useful. Here are a few shortcut keys while in Google plus.
Q open chat
K scroll up within a single post
J scroll down within a single post
Return or Enter Start a comment
Tab and Return End a comment
Space Bar Scroll down your stream