NoScript
From OLSEncyclopedia
NoScript is a Firefox extension that lets you selectively allow individual sites to run Javascript, Flash, Java, and any other embedded objects. It also prevents Cross-Site Scripting, whether it's actually an attack or not. Some sites, like eBay and PayPal, depend on Cross-Site Scripting, which makes that kind of annoying.
As opposed to being a blacklist of sites you don't want to run this stuff, NoScript disables it globally and gives you the option to add a site to your whitelist, selectively allowing that one site (or even subdomain!) to run scripts and embedded objects while the rest are safely blocked.
Also, a really nice feature is that if there's an embedded object on the page, NoScript covers it with its logo, and clicking that allows you to temporarily allow just that embedded object to run.
Being a Firefox extension, it naturally requires Firefox to run.
