This post will provide you the shortcut for the most popular IM, Google Talk. This will enhance your Geeky capabilities. All in all these all 18 shortcuts which I m going to provide you.
1) For writing anything bold, just include that word into asterisks e.g. *Go Hi-Tech* will become Go Hi-Tech
2) For writing anything in italics, just include them in underscores e.g. _Go Hi-Tech_ will become Go Hi-Tech
3) CTRL + Mousewheel up/down will change the font size in a conversation window.
4) CTRL + 5 For 1.5 spaced lines
5) F9 Send an email to the contact you’re chatting with
6) CTRL + L: Left justify text
7) F11- Start a call
8) F12- Stop the call
9) ESC- Close the current window
10) For minimizing the current window press ALT + ESC.
11) Some smileys are converted by Google Talk: , :-O, 
, 
, 
, 
, , B-).
12) CTRL + 1 For single spaced lines
13) CTRL + 2 For double spaced lines
14) CTRL + E: Center text
15) CTRL + R: Right justify text
16) F11 Start a voice chat with the contact
17) ESC Close the current window
18) ALT + ESC Close all Google Talk windows
Hey one more gtalk short cut:
Windows + ESC – Open Google Talk (if it’s minimized, or in the tray).
What about underlining text in Gtalk ?
@ Anonymous, I have tried it, but haven't found it yet. I will post when I find it