I've taken a list of English words, and examined each of them to see what they sound like (using the PHP metaphone function). This page lists the most common metaphone codes, along with their words. You can also search for a word of your own, and see which words sound similar - and which ones rhyme, and which ones alliterate.
goop goopy coop kop gap cop gape gawp gapy coup copy cope copay coupe coypu gappy guppy koppa
hoop coop goop loop poop whoop op