Popular Speed Test Sites:
speakeasy.net (Requires Flash)
testmy.net [Click "Download Speed Test" then "SmarTest"]
DSLReports.com Test for DSL and Cable (Points to speakeasy, BroadStar, net@ccess, ...)
DSLReports.com Java Tests for high speed nets (Points to speakeasy, BroadStar, net@ccess, ...)
DSLReports.com Tests for mobil devices (phones, ...)
SpeedTest.net

Info
DSLReports.com/ ranking of ISPs
reviews-zdnet.com.com/Broadband_Watch/

Bandwidth Speed Test Site List

Note: My tests showed results varied by which test service you used (20%), the media (fiber vs coax [I didn't test DSL]) (50% for some test services), depending on what computer you used (800 MHz PC vs 1GHz Mac) 60%, time of day (3-5%), and which browser you used.
e.g. IE produced the same results except for use with TestMy.net on a Mac where it was 70% slower.

Note: Speakeasy consistently found my Optimum Online (cable) download speed about 20% faster than Verizon FiOS (fiber), while Broadstar and TestMy.net found it about 15% slower. Both were around 10 Mbps download.
On the other-hand upload speeds were 20% slower for Optimum Online with Speakeasy while cable and fiber were the same for Broadstar and TestMy.net; about 1.7 Mbps.

Above tests were done on a Macintosh with OS X 10.4 and a 1 GHz G4 powerbook.
Tests done on a PC with a fresh copy of Windows 2000 on a Dell 800 MHz Pentium III produced speeds 60% slower except for SpeakEasy over verizon fiber which had speed similar to those on the Mac. SpeakEasy over cable service showed speeds 50% slower on the Dell than on the Mac.

The browser used Internet Explorer (IE), Firefox or Safari (Mac) didn't make any difference except for IE on a Mac with TestMy.net where it was 70% slower.

at&t - SBCglobal.net/dsl/speedtest/ Select location in CA, TX, IL, NY, WA, VA
      (upload & download speeds)

at&t/SBC gave results between speakeasy and test my.net on cable (10 Mbps) and very low results (1-2 Mbps) on Verizon fiber.

Browser Performance: Browser Cable fiber Mac PC Mac PC IE Firefox Safari See Also:
Network Performance
ISPs under Data Communications Interfaces