The image on the left is a volume rendering of a bonsai tree. I used a not-very-realistic transfer function which maps colors to density values and linearly interpolates between these colors across the entire volume.

The image on the right is a "bucky ball". I wasn't really sure what one of these things was supposed to look like, so instead of interpolating colors through the volume I assigned a random color to every value.

Made in XNA starting with the tutorial here.