Broadband Optimizer for OS X claims to optimize your network settings for broadband connections (like cable modems) to the Internet. There's some discussion of this program and the results obtained with it on the Ars Technica Networking Forums.

[Editor's note: I downloaded and installed this package. It's a very benign installation - a folder which sits in your /Library/StartupItems folder, and sets four different TCP values. To remove it, just remove the folder and restart. If you have a broadband connection, it can't hurt to give it a try. My test results were interesting. I ran the speed tests from bandwidthplace.com, and I also looked at Quake3 ping times. My download speeds seem to have dropped slightly since installting the package, but my Quake3 ping times are notably (and repeatably) lower. I'll keep an eye on it over the next few days and see how things go with both downloading and Quake3 before I decide whether I'll keep this package or not.]