You can use the following keys when you are not typing in the tweet entry field

key description
C show conversation with the currently selected tweet
D send a direct reply to the currently selected tweet - so that only the receipient can read it
F start sending some feedback to us on what you think of gtwit
H go to the home page of the user who sent the current selected tweet
L shows the links for the user who sent the current selected tweet such as their home page or twitter page
K show a list of the current key bindings
R reply to the currently selected tweet
S opens the search dialog
T go to the twitter page of the user who sent the current selected tweet
U when on the search results page go back to the current tweets view
space bar move up in the tweet selections as you read them
cursor keys move the currently selected tweet
escape closes a popup window such as when you hit S to open the search popup