According to analysis based on the latest Times/YouGov opinion poll the parties will have the following number of seats after the election:
20 Liberal Democrat
With 650 seats, an outright majority requires 325.
Based on these figures the Conservatives will not be able to form a government. Even if they could form a coalition with the Liberal Democrats again and bring in all the "others" plus UKIP it would not be enough:
Conservative (265) + LibDem (20) + UKIP (2) + Others (24) = 295
Labour, on the other hand, would have a far easier job:
Labour (290) + Others (24) + LibDem (20) = 334
So even if all the 24 "others" did not support them they would still have a majority without having to talk to the SNP at all.
Given the historical enmity between Labour and the SNP and the fall out from the referendum it seems fanciful to me that they would make any sort of agreement after the general election. The SNP might choose to vote for policies put up by Labour, but Labour will not need to do anything to keep the SNP on-side as they will not need them to have a majority and get Ed Miliband into No 10.
What makes this even less likely is that the level of support for the SNP suggested in this poll is unprecedented. The likely outcome will be far fewer SNP MP's and more Labour ones, meaning even less of a need for anyone to do a deal. A figure of 35-40 SNP MP's seems far more likely given that younger people are less likely to vote than older.
We can be sure that the Liberal Democrats will want to stay in power, even with a vastly reduced number of MP's. Labour has been careful not to single them out in their national campaign. One of the reasons for this will be their potential as coalition partners on May 8th.