Online rhyme dictionary/rhyming resource that lists rhymes by vowel sound (assonance)

Solution 1:

Here's a link that may help:

http://www.wikirhymer.com/

It's a little weird to use. You have to select a word from the dropdown list. As it says, you have to
". . . type a word into the Find Rhymes box. Select a word from the dropdown list and you will be whisked to the appropriate set of rhyming words including all near and 'pure' (also known as hard) rhymes."

They've made it much easier to use now. Just type a word in the Search box and you can look for its "pure", "end", "near" or "near end" rhyme.

Solution 2:

Try http://www.prime-rhyme.com. It might have the functionality you asked about if I understood it correctly.