Galeras in Colombia is one of the most active in the country and has seen regular eruptions for the past 1 million years. With 7 eruptions since the turn of the century and one occuring in 2010 the activity hasn't stopped since. The 2010 activity caused the evacuation of 8000 people from its vicinity. A nice overview of the area is available here at the NASA Earth Observatory.
From the GVP page for Galeras.
Galeras at ~4276 m high is a stratovolcano which towers over the town of Pasto and produces predominantly andesitic products. The reason for volcanism at this location is the subduction of the Nazca plate. A nice diagram of this is shown here. The summit of Galeras is complex as it has had a diverse and extremely interesting history, involving a couple of caldera forming events, one which occurred 500,000 years ago and the other around 100,000 years ago. In addition to these events a series of edifice collapse episodes have left a horseshoe shape at the summit.
Since the first historic eruption was recorded by the Spanish in the 16th century, the volcanism has been lightly explosive, producing ash clouds and pyroclatic flows in what can be described as vulcanian to sub-plinian events.
This volcano became infamous in 1993 when it sadly killed nine people (six of who were scientists) who were in the summit crater at the time of the eruption. Prior to this sad event, Galeras was designated a Decade Volcano in 1991.
Other Volcano News
Today is the 20th anniversary of the eruption of Pinatubo in 1991. Over at the Eruptions blog by Erik Klemmeti there is an excellent anniversary post, I reccomend you check it out!