It rocks!

M & E is a rhyming dictionary. What this program does is look up definitions of words, but look for words with similar endings.
It's free, has more than a million entries and even allows you to find meaning in the dictionary of the RAE with one click.

You know rappers, for those days of lack of inspiration when you to stay stuck in a rhyme here's M & E:

http://www.mollyandedu.com/merimas/