Partial decriminalization/minimal criminal prosecution: England, Denmark, Slovakia, Latvia, Croatia, Poland, Austria, Germany, France, Netherlands (see chart in the Economist story linked below--it's interesting that the Netherlands has the highest percentage of prison outcomes on this list)
Unconstitutional to prosecute people for drug possession (any drug) per Supreme Court ruling: Argentina, Colombia
Marijuana decriminalized: 14 U.S. states (Alaska, California, Colorado, Maine, Massachusetts, Minnesota, Mississippi, Nebraska, Nevada, New York, North Carolina, Ohio, Oregon)
States with some localities that have decriminalized marijuana: Arkansas, Illinois, Kansas, Michigan, Missouri, Montana, Washington, Wisconsin
Considering marijuana legalization: California, Massachusetts, possibly Oregon
Considering decriminalization (any drug): Brazil, Ecuador
Source: The Economist, "Virtually legal," November 14, 2009; state decriminalization details from Wikipedia.