Sometimes, all you need is a simple tool to get the job done. If you find yourself constantly switching back and forth between browser tabs, a simple bookmarklet might be just what you're looking for.
Dual View is a bookmarklet that loads two web pages side-by-side in one tab.
Drag this link to your bookmarks bar, then launch it from a webpage to execute. A dialog will pop up asking you which url to load in the first (left) pane, then the second (right) pane (both are preloaded with the current page's address).
Once you've input your urls, the screen is split- simple. There are browser extensions that do this, but why install a resource-hogging extension to do a job that a simple, resource-less bookmarklet can do?
Tip: this is especially handy if, like in the screenshot above, you use Instapaper or Readability's decluttering boomarklet to strip an article down, then load notepad.cc in the second pane to take notes.
Here's that bookmarklet again.
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