Slow Internet on Lion, but only for some sites and some browsers
Try renaming /Library/Preferences/SystemConfiguration to /Library/Preferences/SystemConfiguration_Backup and restart your computer. I've had some really strange issues where the network .plists that are stored in SystemConfiguration have caused problems.

If it doesn't resolve the issue just delete the folder and set the SystemConfiguration_Backup back to its original name.

The folder houses config files for some system services, but when you restart it will create new copies of these files.