All the major browsers--Firefox, Internet Explorer 8, Safari, and even the new Google Chrome--now have or will soon have a major privacy feature. The name varies from browser to browser, but the basic intent is largely the same: While active, it masks your browsing trail from anyone who might later sit down at that PC and try to check the browser's history, cookies, and other data.
While these features should prove effective in that situation, they won't disguise you from the sites you visit; sites will still be able to record your IP address and monitor the searches and other information you enter. And more important, the technology won't make you any more or less safe from Web-based attacks that might surreptitiously attempt to infect your PC with malware when you view a page--the favored approach among money-minded digital crooks these days. So you'll still need to be careful where you surf.
Safari's Private Browsing feature comes with its current 3.1.2 release version; for IE 8, you'll need to grab beta 2 to use its InPrivate option. The feature is called Incognito in the recent Google Chrome beta release. Firefox is set to add its Private Browsing to the upcoming 3.1 release, but at press time only the feature specs were available.
In general the functions work (or will work) in a similar fashion. You start by telling the browser to initiate a private browsing session, which for IE 8 and Chrome means you'll get a new window with clear visual indicators that you're in a private session. Safari, unfortunately, uses the same browser window with no visual cues (after an initial pop-up confirmation), so you might easily forget you're in private mode--or think you're in the mode when you aren't. Mozilla isn't saying whether it plans to use a separate window or other visual indicator for its feature.
In private mode, the sites you visit won't be added to the browser's history. When you leave the mode, the browser deletes any cookies added during the session, and clears the download list (or, in the case of Safari, the list won't acquire new entries in the first place). However, downloaded files will stick around unless you manually delete them, as will bookmarks you add yourself.
Sites you visit during a private session generally won't be able to access cookies, history, or other browser data created or saved before you entered the session. IE 8's InPrivate has an additional blocking option that will prevent sites from sharing data about your visit with third-party data collectors, such as ad networks, that the browser learns about during the course of your surfing. But no browser can prevent sites from tracking your visit. To hide yourself from sites, you need to use a service such as the for-pay Anonymizer or the free Tor.
These new privacy features are all worthwhile, and they should all fit in with your everyday surfing much better than existing browser options (such as those that wipe out your entire browsing history). Just keep in mind that they're not a panacea, and that they're for secrecy, not additional safety.