As a Liberal, I'm not comfortable with the discussion of uniting the left. We try to "unite the left" when we compete with the NDP, Greens and Bloc in elections. The "unite the left" movement seems like an undemocratic way to try to achieve the same thing.
I don't see evidence that the NDP wants to be "united" with the Liberals. Since the Liberals are by far the larger party, union would mean killing off the NDP. The NDP has a proud heritage and is the party that brought us many of the social programs that give us the most sense of pride as Canadians - such as universal health care - and if they survive, they will continue to be the party that pushes for progressive policies (as evidenced by their deal with Paul Martin that resulted in a national day care program, later killed by Harper).
Harper didn't unite the right: he staged a successful hostile takeover of the Progressive Conservative party and coopted their brand. People like to say that the coup rebounded on him because the moderate PCs have diluted some of the ideology of the Reform Alliance. That's not really true. Harper runs a tight, top-down, centralized party and his central core calls the shots. With just a minority government he has already made enormous changes to Canada that no real Tory would have made, like moving towards an American-style justice system that aims to incarcerate more youth and wants to create a false culture of fear.
But following along that argument, some would say that uniting the left would result in a greater progressive element in the Liberals. I question this. For one thing, the Liberals already have a very strong progressive bent, and when it is not realized that is because of prudent considerations such as (1) fiscal responsibility and (2) considering all Canadians, not just an ideological base. The prudent nature of the Liberal party is our greatest strength and what makes us stand apart. Some people may malign it as "political expediency" but it is actually what makes us the most effective, decent and responsible governing party, and it must remain our core.
In our recent election, the "left" got 61% of the vote and the Conservatives got 38% of the vote, yet the Conservatives formed the government. If we want to unite the non-Conservative parties, we should think of means that don't involve killing off parties: coalitions and special agreements. These can't be imposed from above, as Elizabeth May found when she suggested strategic voting during the election and disaffected large portions of her party. We need an open dialog. We could start small: greater coordination to achieve common goals during this minority government. Or don't run opposition against each other's leaders, thus freeing up the leaders to do a national campaign (I think there's historical precedent for this but am not sure). In addition, voters could engage in more effective strategic voting.
The problem with strategic voting is that voters need accurate polling information to make a useful strategic vote. Sites like Vote for Environment actually damage our ability to vote strategically because the data is so shoddy. The site claims that "This site offers comprehensive, up-to-date riding by riding information on how to defeat Harper and his anti-environment policies." That implies that they have riding-level polling data. However, in the small print they admit that their numbers are calculated by taking "the number of votes each party received in that riding in 2006 modified by each party's current standing in this week's cumulative polls." In other words, their information is garbage. And it had bad results. For example, Vote for Environment said that Kitchener-Waterloo was a safe Liberal seat and told voters to "vote with your heart." The Liberal lost by a small margin, and Vote for Environment could be considered the reason.
It is definitely unfortunate that the Conservatives were able to form the government with such a small percentage of support (21% of eligible voters) and it's natural to think that the fractured left is the culprit. However, a multi-party left is also a source of strength. It leads to a culture of ideas that the Conservative monolithic structure does not generate. While they are stagnating, we can enjoy a renaissance.