I was recently messing around in debug mode and found out you can spawn liquid tar and water. Surprisingly enough it actually behaves as a body of liquid: It flows down the slopes, it fills the pits (the natural ones and the ones you dig), and it is FINITE. Or at least looks finite. It also has current texture for the places it is flowing down.
It doesn't seem to propagate over the chunk borders (or just has a distance limit). But I think it could be made into real rivers instead of the sea-level canals that we have now instead of rivers.
Was anyone checking if that can be done as a mod maybe?