Let me say right off the bat, I am very uncomfortable with any discussion of 'strategic voting'. Not with the practice itself, but with people and especially candidates try to actively persuade voters to do it. In fact, the last time a candidate suggested something like that to me... well, I didn't react very well.
At the same time, I am not so naive as to think that everyone just voting for the candidate or the party they agree with the most is necessarily going to get the desired result - especially when you're dealing with our multi-party, first-past-the-post system. And especially when you're dealing with somebody like Stephen Harper.
As far as I'm concerned, it's up to each individual to decide whether or not they consider Harper to be enough of a threat to justify voting for a party that might not be their first choice. However, if you are considering a strategic vote this time around, there is an incredibly useful website called Vote for Environment that provides exactly the kind of detailed, riding-by-riding information you should have.
What Vote for Environment has done is identified potential swing ridings, analyzing likely results based on the previous election results adjusted by current poll results, and then recommending the candidate that has the best chance of beating the Conservatives - whether they be Liberal, NDP, Green or Bloc.
In this way, you can see whether or not your riding is 'safe' for any particular party, and if not, which party has the best shot at taking it. It's considerably more precise that just saying "don't waste your vote on ____" because it shows you exactly what the risks and benefits are for your particular riding.
Here is their argument. I'm not saying I agree with it 100%, but we live in extraordinary times and it's something that needs to be considered.
(big H/T to Impolitical)
UPDATE: Scott Tribe brings news that Avaaz.org is putting together a targeted 3rd party advertising campaign pushing for strategic voting in three close ridings - one of which is John Baird's. And their fundraising acumen is evident in the fact that, within 8 hours of launching the campaign, they've raised over $40,000 of the $50,000 they're aiming for.
Also, thanks to commenters here and elsewhere who have put me on to several other strategic voting sites:
Anyone But Harper
Department of Culture (these are the folks organizing protests in swing ridings like the one in Oakville)
Democratic Space Strategic Voting Guide
Vote for Climate (possibly related to Vote for Environment - very similar)
I still thing 'Vote for Environment' has the best, detailed information on specific ridings.
Also, before the NDP supporters start piling on, let me point out that according to Vote for Environment calculations, real riding-by-riding strategic voting would actually result in a 12 seat increase for the NDP, as well as four more Bloc seats and at least one Green MP.
Ok, so I'm rapidly becoming a convert here. So sue me.