The American Civil War - Both the North and the South had democratically elected governments
War of 1812 - US and UK = democracies
World War I - Germany, although arguably not a "proper" democracy by todays standards, it still had a democratically elected parliament (or whatever they call it there) vs Britain, France both of which are democracies
World War II - Germany (Hitler was elected to power) vs Western Liberal Democracies
Kosovo 1999 - NATO (a collection of democratic countries) vs Federal Republic of Yugoslavia a democracy.
2008, Georgia vs Russia
Although it may have some merits, the democratic peace theory is full of shit.