Zack uses a proxy server to examine the page you're looking at and add information. If you subscribe to the free service, you'll be able to bring up the Zack Bar from an icon on every Web page. One tab, Portal, will include headline news and access to search engines. Another, Community, will search newsgroups, translate pages from one language to another, and suggest other sites you might want to visit. The Finance tab, in addition to obvious things like financial news, will have a stock lookup for the site you're currently viewing.
Trivia does pretty much what the name implies.
The site you're on will not have to be one that's cooperating with Zack Networks. Any service that watches where you go and what you consider buying has the potential to create some pretty thorough profiles. Zack Networks promises not to make use of that potential or to sell your data. To its credit, the registration process requires no information other than a username and a password. Yet the service does track merchandise you examine online, in order to provide a product list for your convenience.