I'm registered Green, and vote Democratic if there is no Green candidate.
The question of the utility of voting for minor party candidates is an interesting one.
Some things to keep in mind:
* A vote is not "wasted" simply because it is cast for someone who does not win. If that were true, people who voted for Al Gore "wasted" their votes just as much as those who voted for Ralph Nader, which doesn't make a lot of sense.
* Unless the vote totals for the two candidates differ by exactly one vote, your individual vote does not affect who wins. The odds of such an exact tie are astronomically small, so by this logic voting at all is an irrational waste of time.
* Not since the Republican party won the white house in 1860 has a minor party attained major party status, and it is not likely to happen in the foreseeable future in the US. However, minor parties have affected policies in very significant ways, as their ideas are co-opted by the major parties and put into affect.
The primary reason for voting, it seems to me, is to inform the political establishment about what the voting public wants. A vote for a minor party actually carries much more information than a vote for a major party, because minor parties tend to have very specific platforms that are not watered down to trying to appeal to as many different types of people as possible.
I do vote for a Democrat if I think the election will be close in my state, and particularly if I think there will be important consequences of a Democratic v. Republican victory. This, too, is for the sake of providing information - when a lot of Greens vote Democratic in a close race, it lets it be known that the Republicans cannot count on winning just because of a divided opposition.