Until recently, Adobe Reader included Yahoo Toolbar as an option to install. Macromedia Flash Player gives users the chance to download Yahoo Toolbar. Now Adobe has signed a multiyear agreement with Google to redistribute Google Toolbar with most free Adobe products.
Adobe says that Google Toolbar is offered for the moment to users who download Macromedia Shockwave Player from Internet Explorer. I can't confirm this, since the upgrade to Shockwave player crashed my browser.