Quick links to recommended Internet speed testing tools:

Ookla Speedtest
http://www.speedtest.net/

The following sites provide more in-depth testing of your bandwidth:

Megapath Speed Test Plus
http://www.megapath.com/speedtestplus/ (requires Java)

ISP-specific speed testing sites:

Speakeasy Speed Test (MegaPath) – can be used with other ISPs as well
http://www.speakeasy.net/speedtest/

XFINITY Speed Test (Comcast)
http://speedtest.comcast.net/