BillMonitor which was the first mobile price comparison service to be accredited by UK comms regulator Ofcom earlier this week. It compares vodafone, orange, T-Mobile, O2 and 3.
I hope more price comparison calculators / tables get approved, which can only be good news for consumers.
Ofcom's approval process involves as they say a "rigorous independent audit", so you should be able to trust accredited services to help you choose or switch to price plans / tariffs that save you money - particularly important in these credit crunch times!
You may also be interested in Ofcom's FAQ for consumers on accreditation of price comparison services generally. The vast majority of cost comparison services etc aren't accredited.
Separately, while not Ofcom-accredited, another service called MobileMeg is back.
This focuses specifically on just comparing mobile data price plans (i.e. charges and costs when you're using the internet on your mobile phone, including WAP, mobile internet, mobile email, chat etc).