A Tale in the Desert was the first game I played that used multiple chat tabs. It worked differently than most games: non-customizable, and each saved recent message history so that it effectively included the game’s whisper and mail systems. A Tale in the Desert also allowed multiple guilds, and each guild got its own tab.
CoX and LotRO have good implementations: send whatever chat you want to whichever tabs you want. I can have a “guild, alliance, and whisper” tab to make sure I did not miss anything during a fight, then another for narrowing information during raids or dungeons.
Guild Wars has the interesting addition that it uses !@#$% as a first character to let you indicate which channel you’re speaking in. It also uses what looks like tabs (but are really chat type/channel indicators) for that purpose. They combine to a suitable way of maintaining the last channel you talked in and indicating it visually.
Guild Wars also ties its party search and chat systems. Adding yourself to LFG (or for trade) sends a message to the chat window. It facilitates trade spam, which is unfortunate, but it neatly solves the problem of how invisible LFG is in most games.