Speedtest.net - Test your Internet connection bandwidth to locations around the world with this interactive broadband speed test from Ookla
NetSpeedMonitor - NetSpeedMonitor is a lightweight Network Monitoring Toolbar for your Windows Taskbar
Testmy.net - Accurately test your Internet connection speed with this powerful broadband speed test. Improve your bandwidth speed with the truth.
FreeMeter - Monitor network bandwidth (C#.NET 2k/XP+). Desktop and Systray graph.
OpenSpeedTest - Broadband Internet Speed Test Unlocked!
NetTraffic - Essential network bandwidth monitor.