It is called Swingometer and can be downloaded here. Personally, I’d ignore the others now, because this one is more fun, simpler, and enables you to keep an eye on particular seats as the election unfolds.
The basic principle is explained in the sheet. Effectively, I am happy (see earlier geek posts) with the idea of Labour losing a flat number of percentage points to the other parties, from their current 36%. It looks like the first number should be 7, therefore.
How to apportion the votes between the other parties? My first rule is: in proportion to their non-Labour vote share in 2005. This hard-codes some tactical voting. My second rule is: add or take away a % of the missing Labour votes to the Lib Dems, depending on how their fortunes are going
You can look up a particular target seat (from 200); it will be highlighted for you, and then you can see where it is in the Swing.
Here is a screen-shot:
But go play, and have a nice evening.