It would depend on the country and circumstances hugely.
In the UK, I'd usually vote Lib Dem but would consider voting Labour or Green if it was necessary to keep the Conservatives, Scots Nationalists, or UKIP out of a seat. I'd also stop voting Lib Dem if the party swung heavily enough to the right that there definitely wasn't a place for people like me in it (basically if it just became a "centre party" rather than supporting liberal ideals that I care about). I have voted Labour in the past, though that was when the Lib Dems suspended their candidate in a council election.
As for other countries... in Canada I think I'd be in a similar position to the UK, though I'd probably be a little happier voting NDP there than Labour here. In the US, I think I'd usually vote Democrat, though I'd consider the Greens if they had a chance at winning something or possibly the Libertarians if I was in a GOP seat where only the Libs had a chance of breaking through (but I'm really quite a long way from the US Libertarians ideologically, so that would be purely a tactical vote, and I'm not sure any such seats exist). In Australia I'd never vote Liberal, given that I don't find their Liberal party, well, particularly liberal. Not sure who I would vote for.
I don't think there are many circumstances at all in which I'd consider casting a vote for a conservative or rightist party. Possibly if I was convinced they were the only way to vote to stop a straight-up fascist winning a seat, or if there was an extremely authoritarian pro-surveillance anti-human rights type leftist party versus a libertarian centre-right party I might be a bit split.
