Companies House - the foundation of company information exchange in the UK: helping business, informing the public and benefiting the economy.
www.companieshouse.gov.uk

Princes Trust - An independent organisation aiming to foster success in young people to start and develop their own businesses. Also hosts a national competition for start-ups to accelerate young business ideas. Free advice for 16 to 30 year olds.
www.princes-trust.org.uk

The Department for Business, Enterprise and Regulatory Reform is the government department responsible for UK business.
www.berr.gov.uk

HM Revenue & Customs - Advice for small businesses and those going self-employed. It's surprisingly helpful and full of useful information.
www.hmrc.gov.uk

Startups - This is aimed at SMEs. It has a clear and simple design and combines news with practical advice. There's plenty of practical advice in a no-nonsense style.
www.startups.co.uk

New Business - A comprehensive website dedicated to new businesses.
www.newbusiness.co.uk