I went on the Mt Ruapehu Crater Lake guided walk in May 2003. It's set in the Tongariro National Park, New Zealand. (More photo albums can be found here.)
The lake emptied out somewhat during the last eruptions in 1995/6, but is back to about 80% full again.
We were given the choice of walking down the way we came up (via the rock-and-boulder-strewn path) or sliding down on our bums... it was a no-brainer decision...