Jotto
In Jotto, each player thinks of a secret five letter word. Then, you take turns trying to guess your opponent's five letter word. If you get it wrong, you'll at least find out how many letters your guess has in common with the correct word.
Repeating letters is okay. For example, if the secret word is "apple" and you guess "popes", that's 3 letters in common (two p's and one e).
Once you enter a word, a grid of letters will appear. Click on these to keep track of which letters you know are (or are not) in your opponent's word. Use smart guesses to eliminate letters and figure out the computer's secret word before it figures out yours!