Netflix has put together its ISP Speed Index. allowing you to compare ISP speeds in your country. It makes for some really interesting reading, and puts the marketing claims of ISPs to the test.
Perhaps not surprisingly, Virgin Media is fastest in the UK. I say not surprising as they are the only mainstream ISP to offer widespread cable access. I use Virgin myself and find their speeds to be impressive compared to my ADSL toting friends and family, but their customer service to be truly woeful.