I <3 My Pipes to the Intarwebs
It’s difficult to find sites that are capable of truly testing high speed Internet connections. Speedtest.net is de facto for tests in this area of the US, but test results are often finickly. There’s a whole slew of problems with this service: 1> web servers were designed to serve, not receive; 2> the host provider may be an ISP that’s oversold; 3> you may be testing during the ISP’s peak time; 4> the testing server may not be tweaked for high TCP performance.
So, I’ve been using Internet2′s list of NDT servers that use the customized Web100 kernel which been tweaked for 100Mbps+ throughput. When your system is tweaked properly, you’ll find much more accurate results than what you’ll see from Speedtest.net’s servers.
Below is the result of a test from a Windows 2008 R2 RC box with tweaked TCP parameters for high throughput:
Sweet. 
Here’s a list of Internet speed test sites I frequent:
PS – FWIW, Verizon has built their speed test site using the same userland client as the Internet2 NDT servers, but I’ve always had trouble getting upload to scale beyond 4Mbps on a Windows box no matter how it’s tweaked. YMMV
