I saw this article today "Beating Harper: NDP + Greens?" about NDP + Greens joining forces being a great solution (since we don't PR) to beat Harper. I look at it more as a solution to reduce vote-splitting, not at a way to "beat" any particular other party.
I also agree that this is a great idea; however, this will never happen. I think a better idea would be to trade ridings. Based on the popular vote numbers from the last election the NDP and Greens could work out some riding swapping mechanism whereby the Greens won't run in x ridings of the NDP's choosing, while the NDP won't run in y ridings where the Greens were strongest. It should be computed mathematically based on last year's popular vote in a very clear and scalable way, so that if the Greens were to suddenly get 10% of the vote next election the number of ridings the NDP would "give" to the Greens would increase proportionately in some way without leading to bickering, only leading to continued proportional representation for both parties. I'm not sure if those two parties could ever agree to this. They all seem to be blind to the fact that we are living in a non-PR system, where running 2 left-wing candidates in the same riding is just stupid. It would be like having 5 separatist partites in Quebec. What a dumb idea that would be. Running Greens and NDP in the same ridings, ridings where one or the other has a slim chance of winning, is equally as dumb, although just less obviously dumb.
One negative side-effect of such a scheme is that it would essentially lessen the motivation for actually implementing PR in this country.
One interesting factoid in the article: "In an article published in Canadian Forum in 1989, former NDP MP Lynn McDonald called on the NDP to be the Green Party of Canada." Probably would have been a good idea. They would have grabbed the Green name ahead of the true Greens, preventing the vote split that exists today, and with the support of labour-votes we could easily imagine the Green party having seats today if Lynn McDonald had her way.
There is lots of discussion over here, which I'm not sure I want to get involved in.