I'm intending to wander out to vote around lunch time as I can then combine it with a short hunter-gatherer type trip to the shops to track down todays feeding delights :-p
If everyone that thought 'my vote doesn't count' or 'why vote Green/UKIP/Respect/BNP/whatever' went and voted it could have a massive impact on the putcome of the elections. Just think if turnout is 30% in your area if everyone else voted Green (or another small party) then they would get in on a massive majority. By not voting you are depriving your party a chance to make a difference and represent your views locally, nationally, or internationally.
It's also important to vote if you are anti-party 'X' to help make sure they do not get into power due to general apathy or (depending on your hated party) retain power to do more damage.
The extreme parties generally motivate their voters a lot better than the moderates do, which is another reason you should go and vote. Don't let parties such as the BNP even get a glimmer of hope they have a future.