Every day at word-flip.com. For example, one of today's is CLAY into MUSH, changing one letter at a time OR making an anagram. In 5 steps. Some very hard, such as CUTOVER into MISPLAY, in 16! All words allowed in Scrabble.
Numbers also for those that don't like words. 15443 into 99809, changing one digit at a time (no anagrams allowed this time, but all numbers have to be primes).
Let me know what you think. Will always be free. Solutions always the following day at noon GMT.