For those who are tiring of Eyjafjallajökull and want to think back to earlier times, the Boston Globe's blog The Big Picture has a special on Mount St. Helens (click to see pictures). The eruption happened exactly 30 years ago. Take a peek. For those who lived through those days, it will remind you of the shock and horror of that eruption.
For me, the most poignant photo was of 30-year old vulcanologist David Johnston in the evening at his camp near what is now known as Johnston Ridge near Mount St. Helens. At 8:32 a.m. the next morning, Johnston radioed a message to the USGS headquarters: "Vancouver, Vancouver, this is it!", shortly before he was killed by the massive eruption of the volcano that also killed 56 others.
Another memorable eruption was Mt. Pinatubo in 1991. This photo gives a sense of how enormous that eruption was.
And there was Krakatoa which Wikipedia reports as "with a Volcanic Explosivity Index (VEI) of 6, the eruption was equivalent to 200 megatons of TNT (840 PJ)—about 13,000 times the nuclear yield of the Little Boy bomb (13 to 16 kT) that devastated Hiroshima, Japan during World War II and four times the yield of the Tsar Bomba (50 MT), the largest nuclear device ever detonated."
Of course, for connoisseurs, the really big eruption was Mt. Tambora in 1815. The eruption that created a global cooling and "the year without a summer".
And the granddaddy of all explosive volcanos in the last 25 million years was Lake Toba which according to Wikipedia "The eruption is believed to have had a VEI intensity of 8. This is the largest known eruption anywhere on Earth in the last 25 million years. According to the Toba catastrophe theory to which some anthropologists and archeologists subscribe, it had global consequences, killing most humans then alive and creating a population bottleneck in Central Eastern Africa and India that affected the genetic inheritance of all humans today."
To get a sense of scale and size, take a look at Wikipedia Volcanic Explosivity Index.