You also have Heinlein's concept from Starship Troopers: only those who have committed a year of their life to serving the common good for no pay (food and accomodation covered) earn the right to vote. If that "public service" was of non military nature (unlike in Starship Troopers), I think it would be an interesting system.
On the opposite end of the scale is the concept of Demarchy, found in several SciFis, where there are no elected politicians, and technology enables everyone to vote on everything constantly.
