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.
seep seepy sepia sepoy spy sap cep zep sip sup sop spa supe cepe soup spae sipe spay soap spue spew sappy soapy soppy soupy
beep veep jeep keep neep peep seep deep weep wheep cheep sheep