Jeff Mesnil whipped up 2 bookmarklets in the space of less than one and a half hours that allows you to split a browser into 2 frames. How does it do that? It loads the current URL in a frameset consisting of 2 frames (both showing the same URL). A simple, yet genius, idea.
Here's a screenshot of a blog entry split vertically:
Notice how it could be useful when cross-referencing something on the same webpage, particularly if it's a long vertical-scrolling page. A perfect use case would be referring back to a long blog entry when making a comment.
Here's the same page split horizontally.
There are probably lots of other use cases for a webpage splitter. Cedric's in particular is this:
Just recently, I was reading an article that showed a figure at the top and which constantly referred to this figure in the rest of the article.