Now that Ralph Nader is once again contemplating running for President, surely the statute of limitations has run on respect for the man's past work as a consumer safety crusader. He has clearly devolved into egomania. Indeed, it seems unlikely that Nader would have any effect on the 2008 election, unless we have a replay of 2000, with the vote in a decisive state so close that a tiny percentage of "spoiler" votes is able to turn the tide. But Democrats---and depending on the Republican nominee, perhaps Republicans---have much more to fear from a Bloomberg candidacy, both because of his ability to spend huge sums of his own money and because he won't be dismissed as a fringe candidate.
The obvious solution is "instant runoff voting," (IRV), in which voters rank their choices for candidates. That way, if in some state, say, the Republican candidate gets 45% of the vote, the Democrat gets 44% of the vote, and the remaining candidates get 11% of the vote, the votes of ballots that gave first place to those 11% are redistributed to their second choice (or if neither of the two major candidates is second, then third, fourth, etc), so that the ultimate winner actually is the choice of the electorate in a head-to-head race between the two most popular candidates. Various U.S. jurisdictions already use IRV, as do many other countries, and unlike some other reforms, there is a decent chance that the major parties could get behind it, because it virtually eliminates the spoiler problem (and there's no reason why spoilers are more likely, a priori, to hurt Dems more than Repubs or vice-versa). To be sure, IRV helps third-party candidates because it allows voters to rank a third-party candidate first without worrying that she is wasting her vote, and for that reason, would help third parties to build support over time. So from the perspective of the major parties, IRV is at best a mixed bag.
Even with IRV, a third-party candidate can still play the role of spoiler by winning one state, and IRV makes it somewhat MORE likely that a third-party candidate would throw the election into the House of Representatives. As noted above, IRV lowers the cost of voting for third-party candidates, so a very strong one (like Bloomberg) could conceivably win a state or two, leaving no candidate with a majority of electoral votes. But that defect is probably best laid at the door of the Electoral College, for which the cure is either a constitutional amendment or the intriguing idea of a national popular vote interstate compact.
Alas, we're probably already too far into the 2008 campaign for these inherently non-partisan reforms to be adopted, because the parties and the candidates will ask whether they will benefit in the next election. But if timed properly, perhaps these ideas could get some traction, especially if there's anything resembling a replay of 2000.