For those that don't know, Jotto is a deductive word game. The best short explanation is "Mastermind with five-letter words." Here are some variants that I have played:
Twisted Jotto - This is the first variant I came up with. In it, each guess you make must be able to be the secret word given the scores from the previous guesses. For example, if candy scored 2 and ankle scored 1, aphid would be a legal guess, but scant would not. After playing this for a while, I recognized that this was getting words faster than other methods. So, the people I play with generally stick to the rules of Twisted Jotto, even when not playing Twisted Jotto. There are times, however, when it's advantageous to deviate from this, as when you know 4 letters and are trying to discover the 5th.
Count Rugen - mattlistener came up with this name for Jotto with six-letter words. Count Rugen is the six-fingered man in The Princess Bride.
JottoZendo - This variant is based on a dream I had. I see it as a cross between Jotto and Zendo. In JottoZendo, you pick a secret word, but you also pick a secret rule. The rule is of the form "Score x (some letter in the secret word) only if y (any letter, not necessarily one in the secret word) is also in the word." So, if the secret word were cable and the secret rule were "Score e only if t", then laces would score 3 (instead of 4 as in normal Jotto), but table would score 4. I have only played this against a program I wrote. In it, I restricted the "enabler" letter to the 13 most common letters of the alphabet. Variations on this could include allowing other letters to be the "enabler" and allowing other types of rules besides "Score x only if y."
X-Jotto - magid came up with the idea for this Jotto variant. The secret word is of an unknown length, from 3 to 8 letters. This is a lot more interesting, as it allows for more variety in words to guess and provides a lot of challenge for people seeking more complexity than normal Jotto.