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.
spay spy spa spae soupy soppy soapy sepoy sappy seepy sepia sip sup sap sop seep sipe soap soup spew spue supe zappy zippy