Speedtest.net isn't your only option for testing your connection; MegaPath (formerly Speakeasy) also offers a speed test, as well as what it calls Speed Test Plus, which tries to test the quality of your connection in addition to its raw speed. Your ISP might also offer a speed test tool, though you may need to do a little poking around for it.
If you want to track your connection's performance on a more ongoing basis, consider installing a utility like NetSpeedMonitor: This free tool for Windows XP, Vista 7, and Server 2003 lives in your taskbar, and will give you information on your connection's upload and download speeds. On OS X, the free MenuMeters utility will do much the same, though it will also track other aspects of system performance.
If your connection isn't as fast as you expect it to be, a lot of different factors could be coming into play. It could be a problem with your equipment, or it could be an issue on your ISP's end. For what to do next, see Lincoln Spector's tips on how to address a slow Internet connection. If your connection is fine but you want to tinker with it to try and get the most out of it, you can try changing your DNS server. This takes a little bit of effort, but with the right tools and a little patience, you can get a nice performance boost.
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