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