We've long known that the theme from Gilligan's Island, "Amazing Grace," and "House of the Rising Sun" can all be sung to each other's tunes. However, that only scratches the surface. From the YouTube page:
The Gilligan's Island theme song is what is called a "Ballad Metre" (or Meter). Each stanza of ballad metre has four iambic lines. Typically, only the second and fourth lines rhyme. Here are 31 short samples of the some of the songs whose lyrics can be sung to the Gilligan's Island Theme Song and Vice Versa (mostly, not every lyric fits perfectly).
Read more about ballad metre here. The YouTube page also has a list of the 31 songs with timestamps so you can skip ahead and hear the ones you want. Some are Gilligan's Island lyrics set to other tunes, and some are other lyrics set to the TV theme's music. You can switch them around in your head if you like. (via Metafilter)