Personal possession of any drug decriminalized: Spain, Portugal, Italy, Czech Republic, Baltic states, some German states and Swiss cantons, Mexico.
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.