Scrabble's got to rank pretty high up there. I do enjoy a casual game of Taboo (if you're looking for word games in general and not just spelling games).

I occasionally play those word games on Yahoo! Games but never "fell in love" with any of them.

I also paly some word games occasionally at : link and i find many free and interesting games at link

Scrabble is the best word game ever made. The best variation of Scrabble is Scrabble Cubes.