Mount St Helens Eruption May 18 1980 (US Geological Survey -- Austin Post)
At 8:32 AM on May 18, 1980 – thirty years ago today — Mount Saint Helens, 100 miles south of Seattle, Washington, exploded in the largest volcanic eruption in the contiguous United States since that of Mount Lassen, California, in 1922.
More than 220 square miles of forest were smashed flat, 57 people were killed, and much of the Pacific Northwest was choked by clouds of gritty volcanic ash. The mountain, previously 9,677 feet in elevation, blew away 1,312 feet of that height.
Photographs of Mount St. Helens as it appears today are featured in NASA’s Earth Science Picture of the Day, along with some commentary on the eruption.
YouTube offers this short video of the Mount Saint Helens eruption (among numerous others).
Update: Boston.com displays an absolutely phenomenal collection of images from the Mount St. Helens eruption here. You really shouldn’t miss it.
See also the US Geological Survey’s collection of Mount St. Helens images.
Another Update: Scientific American features an interesting slideshow on 11 Surprising Natural Lessons From Mount St. Helens.
Yet Another Update: Here’s one resource I failed to note that is of significant interest, the US Geological Survey’s Cascade Volcano Observatory page on Mount St. Helen’s 30th Anniversary.
In particular, note the USGS poster of 30 intriguing facts about Mount St. Helens, a couple of downloadable videos (6 to 7 minutes in duration), and this photo archive, and a great before-and-after pair of photos, among sundry other items of interest.
Before and After:
Mount St. Helens One Day Before Eruption (USGS)
Mount St Helens Two Years After Eruption (USGS)
Haysville Community Library 
How cool is Mount St Helens, its as almost as good as my town in the UK (St Helens)