http://petitions.pm.gov.uk/
Petition Tony Blair, and get as many people as possible to back you up.  I doubt he reads even the most popular ones, they’ll all be handled by some flunky, but it feels like being part of the decision making process.

http://petitions.pm.gov.uk/IDcards/ Back the scrapping of ID cards.  Closes soon, so make your voice heard.

Less seriously- http://petitions.pm.gov.uk/Sextoysandcigars/ cut the tax on sex toys, lingerie and cigars.  You have until Sunday to back this one.

Not sure if this one’s serious or not – http://petitions.pm.gov.uk/TheAlcopopBill/.  "Force the manufacturers of Alco-pops include a high dose fertility control hormone in order to reduce the rate of un-planned teenage pregnancies in the UK"

I’m got an idea for a petition.  Something along the lines of "Introduce a Cycling Proficiency style test for children and adults to make them more confident and competent on the road.  This will be voluntary except for- bus and taxi drivers, officials with responsibilities for transport, those guilty of certain driving offences and motoring journalists.  Include a section on cycle craft into the driving test which will be waived for any learner who has passed Cycling Proficiency in the last five years.  This will make motorists more aware of other road users, improve the safety of cyclists on our roads and hopefully increase the number of people using bikes."