First there's the discovery of the Wumpus game. By firstname.lastname@example.org to your buddy list you will be able to play the classic text-based game.
Adjusting your Google Talk font size is possible, you need to hold the ctrl key while moving the scroll wheel of your mouse up or down. This tip works while being focused in either the read or write area of Google Talk.
Just like in MSN Messenger (and others?) you are able to insert linebreaks be holding shift and hitting enter.
Primitive text styling is also possible. By placing * around text you can make it bold and by using _ you can make it italic. (Example: *bold* will return bold and _italic_ will return italic)
Switching windows can be done by hitting tab. It will select minimized conversations, to expand them just hit enter. If you just want to cycle through IMs and don’t care about the buddy list, control-tab will do that and will automatically expand a minimized conversation if you settle on one.
And here is an additional trick I found over here. Google Talk supports some smileys like or :-(. But it won't return a graphical emoticon, it will just turn them blue.