Anthony Wells has published an SVG swingometer on the UK Polling Report blog.
It comes in two variants, UK-wide and advanced (the latter makes it possible to implement regional swings).
It’s really good fun to play around with, and it’ll be a great tool during the general election campaign.
PS: If you don’t know what SVG is, read this (but it’s just a graphics format, so feel free to remain blissfully ignorant, too).