From SpeedDownload website:
IV. Uninstalling Speed Download:
Do not simply trash the Speed Download folder to uninstall Speed Download.
If you are using the latest version of Speed Download, you will see an uninstaller included in your Speed Download folder. This uninstaller will properly remove Speed Download from your system and restore your configuration to how it was before Speed Download.
MANUALLY UNINSTALLING Speed Download from your computer:
Under Classic: Drag the following to the Trash:
- The Speed Download folder
- Speed Download CMM (found in: System Folder/Contextual Menus)
- Speed Download Browser Plugin (found in: System Folder/Internet Plug-Ins)
- Speed Download Preferences (found in: System Folder/Preferences)
- Speed Download Queue (found in: System Folder/Preferences)
- SD Downloads folder (found in: Documents)
Under MacOS X: Drag the following to the Trash:
- The Speed Download folder
- Speed Download Browser Plugin (found in: User/Library/Internet Plug-Ins)
- Speed Download Browser CMM (found in: User/Library/Contextual Menu Items)
- Speed Download Preferences (found in: User/Library/Preferences)
- Speed Download Queue (found in: User/Library/Preferences)
- SD Downloads folder (found in: User/Documents)
Open your browser's preferences, and go into the 'File Helpers' section.
Delete all entries that contain Speed Download.
Quit and restart your browser.