Firefox: Many of us probably have extensions, bookmarks or other shortcuts to deal with URLs, whether it's checking their trustworthiness or shortening their URL for Twitter. Flagfox adds a whole slew of these features to Firefox's address bar.
Flagfox's main purpose is to look at the site you're visiting to find its location, IP address and other information. However, it has a lot of other useful features more relevant to your day-to-day life: with just a click on its icon, you can look the site up on SiteAdvisor, check its Web of Trust scorecard, see if it's down for everyone or just you, shorten the URL, translate it on Google Translate, or even looked up Google's cached version of the page. You can pick which options show up in the dropdown from the app's preferences, and even give certain actions a shortcut (so, say, a double-click on the icon triggers URL shortening). You can even create your own custom actions. Hit the link to see more.
Flagfox is a free download, works wherever Firefox does.