Original idea from PieIsGood, rules and logistics worked out by Yoss and TinTin.

First player posts a song lyric (like in Time Waster Karaoke).  The next player changes a few words in the first line, making sure to preserve any existing rhyme scheme, and then marks the line with >.  Players continue mutating lines until all lines have been modified.

The next player then posts a new lyric and we start all over again.

Example:

Player1:
Row row row your boat
Gently down the stream
Merrily merrily merrily merrily
Life is but a dream

Player2:
>Float float float your boat
Gently down the stream
Merrily merrily merrily merrily
Life is but a dream

Player3:
>Float float float your boat
>Gently under stream
Merrily merrily merrily merrily
Life is but a dream

Player4:
>Float float float your boat
>Gently under stream
>Ha ha fooled you all
Life is but a dream

Player5:
>Float float float your boat
>Gently under stream
>Ha ha fooled you all
>I'm a submarine

Now you post a new lyric.

Imagine there's no Heaven
It's easy if you try
No hell below us
Above us only sky
Imagine all the people
Living for today