year and site

in ecology year a study is done in, and the site it is done at, are
obvious random effects.

You're picking the years at random? - that's a pretty impressive study design!

As I'm sure you realize, Hadley, it is the effects that are random,
not the years.  If I have studied a particular location for the last
three years and I use a fixed-effect for the years in my model then I
can "predict" the response for any one of those three years that I
wish to.  But I have no information on which to base a prediction for
next year, which is usually what I want to do.  (I am assuming the
year is being modeled as a categorical variable rather than in terms
of trends.)

Using random effects for the year allows me to characterize the
variability between years and that does help me in characterizing the
variability for the next year.