I think the Cardassian Union (after the coup on Cardassia, DS9: Way of the Warrior) was a democracy, not to mention a few democracies in the Delta Quadrant that Voyager met.
The Ferengi Alliance was a form of constitutional monarchy. They had a congress, with the Grand Nagus as head of state. The Rules of Acquision and Bill of Opportunities acted as a sort of constitution.
The Bajoran Provisional Government is a democracy.
A lot of planets that only appeared in one epsisode, such as Ventax II, appeared to be democratic.
The Borg Collective is probably the purest form of democracy, even though they are brainwashed. Borg drones are not "brainwashed", assimilated beings have no free will.