The Alaska Volcano Observatory tells us that there are 130 volcanoes in Alaska. Since the 1700s, some 50 volcanoes have erupted and are called active. Most active volcanoes in Alaska are found along Cook Inlet and down through the Aleutian Islands. Inactive volcanoes, those that haven't erupted for several hundred years, can be found in every region of the state.

A few facts about Alaskas volcanoes:

• Alaska has 80 percent of all the active volcanoes in the United States and 10 percent of those found in the world.
• When Mount Novarupta blew its top on June 6, 1912, it was the largest eruption ever recorded. Nearly 100 years later, it is still the largest. Located in what is now Katmai National Park and Preserve, the eruption dumped 700 feet of ash in a 40 square mile area that is called the Valley of Ten Thousand Smokes. [link to sidebar on this eruption]
• For many years Alaska has averaged at least one volcanic eruption per year.
• Shishaldin Volcano erupted nine times in the last 20 years. It has blown every year between 1993 and 2000. That makes it the most active volcano during that time period. It is near False Pass on the far eastern end of the Alaska Peninsula.
• The eruptions of Mounts Spurr in 1992 and Mount Redoubt in 2009 caused trouble for more people than all of the eruptions of Shishaldin. Spurr dumped several inches of ash on Anchorage. People were still cleaning ash out of their rain gutters years later. Redoubt's ash plumes cancelled air travel for thousands when it was most active in late March/early April 2009.
• Fourpeaked Volcano erupted in September 2006 for the first time in 10,000 years, proving that long sleeping volcanoes can wake. http://www.avo.alaska.edu/activity/Fourpeaked.php

Eruptions since 2000:

• Augustine, 2005
• Cleveland, 2001, 2005, 2006, 2007, 2009
• Fourpeaked, 2006
• Kastochi, 2008
• Korovin, 2005, 2006
• Okmok, 2008
• Redoubt, 2009
• Shishaldin, 2000, 2004
• Veniaminof, 2002, 2004, 2005, 2006, 2008

Sources:

http://www.avo.alaska.edu/ 
Alaska Almanac. Alaska Northwest Books, 2008
Alaska's Volcanoes, Alaska Geographic, Volume 18, Number 2
Alaska's Natural Wonders, Bob Armstrong and Marge Hermans, Alaska Northwest Books, 2000