There have been four times in the nation's history when the winner of the popular vote was not the winner of the electoral college vote. The electoral college was established in Article Two of the Constitution, and has only been modified twice, with the ratification of the 12th and 23rd amendments. However, the previous three times that the winner of the electoral college wasn't the same as the winner of the popular vote, were all in the 19th century: in 1824, 1976, and 1888. The 1888 election is the one historians feel the circumstances and issues surrounding the election most resonated with the circumstances and issues surrounding the 2000 election.
There has also been one tie in the nation's history: in 1800 between Jefferson and Burr, who ran a coordinated campaign to oust the incumbent John Adams. Their tactics worked, and the election was decided by the House of Representatives, as the constitution mandates in the event of an electoral college tie.
However, in the contested election of 1876, the House did not decide the election, because of bitter divisions between the House and Senate over the ambiguous "instructions" in the Constitution of how an election should be resolved when one candidate wins the popular vote, and another candidate wins the electoral college vote. So it is the 1876 election, historians say, where the circumstances of the electoral college debate over the results, most resonated with the circumstances of 2000. Hell, Florida was embroiled in voter fraud charges in 1876 too!
beardedbruce is referring to a "faithless elector" I believe (one who pledges to vote for a particular president and vice president, but then casts their vote for someone else). This has been going on for a very long time. Though it isn't common, it has happened numerous times. None have ever played a role in deciding the election though, and I highly doubt that would be the case this year either.
The electoral college system is really an antiquated patronage system, pure and simple. According to Wikipedia, until the 19th century, the concept of an authentic democracy ruled by the entire populace was regarded by the ruling elite as mob rule, and political parties viewed with suspicion. If any of you have read the Federalist Papers (which I rather doubt), you would know that the "founding fathers" never intended direct election of the president and vice president by the popular vote, and presumed that most presidents and vice presidents would be elected by the House of Representatives (they are designated to poll each state delegation for one vote each in the event of a tie in the electoral college).
The electoral college need not even poll it's state's popular voters! The electors appointed by each state's legislature can vote any way they wish to, and sometimes do. South Carolina never held popular votes for president and vice president until 1860.
But then, the "founding fathers" never intended that the presidency would usurp the Congress' power, which is the reality we are living with now.
Many people feel the power of the executive needs to be checked and more legislative power restored to the Congress, more than the electoral college needs to be changed. Personally, I feel both needs to happen, along with reform of the campaign finance laws (ie anybody can contribute whatever they want, but that the money gets doled out to candidates by a non-partisan institution and not the political parties. And finally, there needs to be reform of the incumbency system of patronage in Congress as well. That would have to include reform of the census districting system that was so well abused in Texas recently.
However, in this era of rule by lobbyists and executive fiat, I'm not holding out much hope of the power of the executive being checked by Congress. Congress is too busy feeding at the trough of comfortable campaign finance incumbency to enact any sort of reforms. That will have to come from the populace. In the event of another contested presidential and vice presidential election, reform may come. But I doubt it. The US constitutional system of elections is much too static and petrified, the career politicians too entrenched in their positions, and the US electorate too distracted to care, for any meaningful change to ever be enacted. I think the US political system will simply continue rotting from within and being corrupted from without by the corporate elite, until one day it is overthrown, it fractures regionally, or some such. One thing we all can count on, it won't last forever, thank goodness.
I for one wouldn't care if there were another contested election like 2000, even though I really don't think it is likely. While I don't agree that the 2000 election was an anomaly in a historic sense (it has happened before--three times before!), I think the Democrats are trying to get as much mileage as they can fear mongering their base over it, in order to turn out their voters (many of whom sat out the 2000 election) on election day.
This political system is pretty rotten and corrupt, IMO, but it is still very strong. It will likely be at least another hundred years before the US as it is today, falls.