Chrome: If you need to hide one of your Chrome tabs in a hurry, the Disguised Tabs extension will move it to the background and change its tab name to the unassuming "Google".
Let's be honest -- we've all been doing something on the computer that we didn't want others to see, whether it was browsing tech blogs at work, buying a gift for a family member, or something more... risky. The Disguised Tab extension for Chrome acts like a Boss Key for your browser: just hit Ctrl+Q and it'll move to one of your other tabs, changing your secret tab's name to Google. We'd love to be able to change the shortcut or the disguised tab's page name, but the extension's young, so hopefully we'll see it beefed up in the future.
It's somewhat similar to previously mentioned PanicButton, except with the big advantage of not closing your tabs. That means if you actually had saved work in your private tab, you won't lose it when you invoke Disguised Tabs -- you'll just hide it behind a different tab name. Unfortunately, though, it can't hide multiple tabs in its current state, so if you need that then you're out of luck.