Pimp.my.Gmail series!
If you prefer a script, check out : http://userscripts.org/scripts/show/75047

This style will pimp up the Gmail chat. It will replace the default smileys with the ones from the tango project.
It will also replace some hidden ones not accessible by the smiley menu ;)

PS: You need to have emoticons enabled for this to work.
If you can point me to better/more smileys than the ones covered here, i'll be glad.

Find the rest of the Pimp.my.Gmail series @ "kilroy_@userstyles"
Also check out some of other Gmail scripts @ http://userscripts.org/scripts

CHANGELOG:
v0.5 : 23-07-2010
- Updated to work with new Chat window layout.
- Fixed a bug which affected non-tango smileys.
v0.4 : 08-05-2010
- Replaced ugly drop-down icon for menu.
- Fully compatible with Opera.
v0.3 : 06-04-2010
- Added a work around for a bug which prevented me from adding the devil smiley.
v0.2 : 26-03-2010
- Optimised the code a bit.
- Added a background frame for