Carnaj

You create up to eight words as long as possible by adding a letter each time. But be careful, the computer is watching: if you miss a word, it confiscates it. It's quite challenging, but it's an excellent vocabulary exercise. And for each game, you can choose the maximum word length: 9 letters (like in Jarnac) or 8 letters (easier). You also have the choice of allowing conjugated forms of verbs or not. When you don't know a word, you can look up its definition.
This is the ideal game for Scrabble fans because it's based on the official dictionary. Finally, you can share your best scores with other players.