Just a quick plug for www.tacticalvoter.net. In seats where the second party has at least 60% of the vote and the Tory majority is under 20% they arrange for Labour and LibDem supporters to meet via email and arrange to swap their votes.
My constituency is listed here and tacticalvoter.net says
Labour are best placed to unseat the Conservative MP here. With the help of Lib Dems swapping to vote Labour, it can be done.
If you're a Lib Dem supporter or a progressive voter who is prepared to vote Labour here and would like to encourage a vote for the Lib Dems elsewhere, you can register to "swap" your vote. We'll put you in touch by e-mail with someone considering supporting Labour in another marginal. You agree to vote Labour, they agree to vote Lib Dem and you both do your bit to keep the Conservatives on the back foot
Couldn't have put it better myself!