Not quite a spelling game, and almost a vocabulary game, Smart Mouth will pick your brain and just might leave you at a loss for words. 

Slide the "Letter Getter" device to reveal two tiles.  Players must come up with words that begin and end with the two letters shown, and the winner takes those two tiles.  By standard rules, the first person to shout out a word 5 letters or longer wins and keeps the tiles.  C-N could be "crayon", "carbon", "collaboration", get it?  For an extra challenge, trump each others answers with longer and longer words until your vocabulary is exhausted – longest word gets the win.  Still too easy?  Try imposing category rules (foods only, famous people, etc). 

Younger players who are just learning to read may be able to make three-letter words, but ThinkFun makes another game, "What’s GNU?", just for that age set.

Fun for anyone 8 and up.

For as many players as you can fit in the room.