  We report on a blind survey for extragalactic radio variability that was
carried out by comparing two epochs of data from the FIRST survey with a third
epoch from a new 1.4 GHz survey of SDSS Stripe 82. The three epochs are spaced
seven years apart and have an overlapping area of 60 deg^2. We uncover 89
variable sources down to the millijansky level, 75 of which are
newly-identified, and we find no evidence for transient phenomena. This new
sample of variable sources allows us to infer an upper limit to the mean
characteristic timescale of AGN radio variability of 14 years. We find that
only 1% of extragalactic sources have fractional variability f_var >3, while
44% of Galactic sources vary by this much. The variable sample contains a
larger fraction of quasars than a comparable non-variable control sample,
though the majority of the variable sources appear to be extended galaxies in
the optical. This implies that either quasars are not the dominant contributor
to the variability of the sample, or that the deep optical data allow us to
detect the host galaxies of some low-z quasars. We use the new, higher
resolution data to report on the morphology of the variable sources. Finally,
we show that the fraction of sources that are variable remains constant or
increases at low flux densities. This may imply that next generation radio
surveys with telescopes like the Australian Square Kilometer Array Pathfinder
and MeerKAT will see a constant or even increasing fraction of variable sources
down into the submillijansky regime.
