UPDATE: In Safari 8, the default Web browser for OS X 10.10 Yosemite, Apple replaced the "Reset Safari" menu item with one called "Clear History and Website Data," which does NOT reset the browser. Users of Safari 8 should try going into the Safari Preferences menu and manually resetting the home page and default search engine, but that may not work against persistent browser hijackers.
If your Apple Safari Web browser suddenly has an unwanted toolbar, its home page has changed without your permission or your search results appear in a search engine you never chose, it may be time to reach for the browser reset button.
Many legitimate pieces of software, especially freeware, slap on third-party, browser-hijacking extensions when you install them. The easiest fix is a full browser reset.
Resetting Safari won't kill your bookmarks, and you can fine-tune which settings get reverted to their defaults. You may want to select them all to make sure you get rid of unwanted add-ons.
1. Click on the 'Safari' heading at the top left of the computer screen.
2. Select 'Reset Safari.'
3. Click 'Reset' in the confirmation pop-up window after unchecking the items you'd like to keep. (The question mark in the bottom left tells you about each item.)