I have been experiencing some odd behavior with my network performance. Mostly issues associated with HTTP requests hanging. I have not solved the issue yet, but I do have a few tools that come in handy for figuring out if your ISP is up to no good.
The first is Glasnost from the Max Planck Institute. The java applet will check for packet shaping antics as well as throttling from your ISP.
Another handy tool is the Line Quality Ping Test from DSLReports.com. Using something like speedtest.net only tells you your bandwidth. For me, bandwidth is almost never the issue since I pay a premium for 40Mbps service. The ping delay can still kill performance. It's also nice to know for sure that I am getting what I pay for.