I know I certainly have, so I YouTubed it. It turns out the song is called "Island Song," and is sung by Ashley Eriksson. Ashley performs in an indie pop band called LAKE, which was formed right here in Olympia. They have released multiple albums on the Olympia-based record label K Records. Pretty cool, right?
Here is the full song version of "Island Song":
Here is the end credits sequence:
Here is LAKE performing "Don't Give Up" from their 2009 album "Let's Build a Roof" in Austin, Texas:
Here is a picture of Ashley at the Cartoon Network studios with topiary of Finn and Jake:
Come along with me
To a town beside the sea
We can wander through the forest
And do so as we please...