I found a great site that I use very often for coming up with words for spelling lists and for phonics and other things. It's called MoreWords.com. You can put in any letter or letter combination in any position in a word, and it will generate an extensive list of words for it.
One thing I recently used it for was making spelling lists of words ending in -able and words ending in -ible for one of my middle-school students.
It's intended for crossword puzzles and word games like Scrabble, not directly for education -- but then, many of the most useful educational tools are things that weren't intended for education, if you're creative enough!