I've been reading "Krakatoa: The day the Earth Exploded" recently - it's a fascinating summary of the huge volcanic explosion of 1883. Apparently timely as well - take a look at these two stories of an eruptions on Sumatra - here and here. These things can be nasty - the Pinatubo eruption in 1991 cooled the planet for a few years, for instance.

Nothing in modern memory compares with the "year without a summer" - 1816. It was popularly known as "Eighteen hundred and froze to death". Here's some basic info from the Wikipedia page:

The unusual climate aberrations of 1816 had the greatest effect on the American northeast and northern Europe. Typically, the late spring and summer of the American Northeast are relatively stable: temperatures average about 20 25 °C (68 77 °F), and rarely fall below 5 °C (41 °F). Summer snow is an extreme rarity, though May flurries sometimes occur.

In May of 1816, however, frost killed off most of the crops that had been planted, and in June two large snowstorms resulted in many human deaths as well. In July and August, lake and river ice were observed as far south as Pennsylvania. Rapid, dramatic temperature swings were common, with temperatures sometimes reverting from normal or above-normal summer temperatures (as high as 35 °C, or 95 °F) to near-freezing within hours. Even though farmers south of New England did succeed in bringing some crops to maturity, maize (corn) and other grain prices rose dramatically. Oats, for example, rose from 12 cents a bushel the previous year to 92 cents a bushel.

All of that was caused by a large eruption in - you guessed it - Indonesia (then the Dutch East Indies). It's too far back to know the cause for sure, but something very similar happened in 535, as recorded by Byzantine historians:

In the years 535 CE and 536 CE, several remarkable aberrations in world climate took place. The Byzantine historian Procopius recorded of 536 CE, "during this year a most dread portent took place. For the sun gave forth its light without brightness ... and it seemed exceedingly like the sun in eclipse, for the beams it shed were not clear.". Tree ring analysis by dendrochronologist Mike Baillie, Queen's University, Belfast, shows abnormally little growth in Irish oak in 536 CE and another sharp drop in 542 CE, after a partial recovery. Similar patterns are recorded in tree rings from Sweden and Finland, in California's Sierra Nevada and in rings from Chilean Alerce trees.

If one of those volcanos in Indonesia goes in a similar fashion, we could have a very nasty couple of years in front of us. Let's just hope that they don't - The people of that region have suffered enough, and the modern world is less well prepared for that kind of disaster than the early 19th century world was, IMHO.