Instant Runoff Voting:
Instant runoff Voting is a voting system used for single winner elections in which voters rank candidates in order of preference. In such an election, if no candidate receives an overall majority of first preferences, the candidates with fewest votes are eliminated one by one, and their votes transferred according to their second and third preferences (and so on) and all votes retallied, until one candidate achieves a majority. The term 'instant runoff voting' is used because this process resembles a series of run-off elections.
Advantages: Allows voter to express preferences regarding all candidates, guarantees majority in races with more than two candidates.
Disadvantages: Unfamiliar to many.
More information about Instant Runoff Voting
Plurality (At-Large) Voting:
In plurality-at-large voting, all candidates run against each other for n number of positions. Each voter selects up to n candidates on the ballot, and the n candidates with the most votes win the positions. Often, voters are said to have "n" votes, however they are unable to vote for the same candidate more than once as in cumulative voting.
Advantages: Easy to understand, attempts to limit negative campaigning.
Disadvantages: Vulnerable to forms of tactical voting.
More information about Plurality (At-Large) Voting
Plurality (At-Large + District) Voting:
Same as above, except that the system also includes some single-member districts.
Advantages: Easy to understand, ensures representation from a crossection of diverse interests
Disadvantages: Districting must be done in a manner all can agree with.