Chrome/Firefox: If you want to know whether something is safe to download, Malware Search lets you right-click any text and instantly search security databases for more information. You can highlight the name of an application or a download and right-click to check it against the Threat Expert, Process Library or Microsoft's Malware Protection Center.
Malware Search's usefulness doesn't stop there though — if you're wondering whether a particular site is trustworthy, or who really owns a URL you plan to visit, you can right-click its name and read its Web of Trust listing, or perform a simple web-based whois to find out who's registered the site and where they're based.
The add-on hasn't been updated in a while, but the databases it connects to certainly have been. Even though it's a bit old, it still works fine in most recent versions of Firefox and Chrome. If you're doing some research — or just like to look before you leap, it's a great tool.