Besides Trillian, now also Miranda IM has Gmail notifier plugin

get it here:
http://miranda-im.org/download/details.php?action=viewfile&id=1473
screenshot:
http://miranda-im.org/download/files/screenshot/58-1-1095004682-s.gif

GAlert – A Better GMail Notifier
Support multiple GMail accounts and GMail Label filters
Support easy automatic login through context menu and system wide Hot Key
Support Google News instant notification

Get it here:
http://www.massgrid.com/content/view/58/153/

screenshot:
http://www.massgrid.com/content/view/64/122/