A tool that will come in handy for designers and people who for one reason or the other must compare multiple websites at once, SitePouch can be used for free, and without the need to register first. It is a web application that lets people add all these URLs that they need to have visualized, and then open all them in the very same window. Each site will be resized in order to fit into its allocated window.
From what I have seen, you can add up to 30 different URLs at the same time. And it seems you can add even more, too. Needless to say, the more URLs that you pile up the less useful the application turns out to be – the windows become too small for you to appreciate things in significant detail.
There is not a lot more to say, other than warning you that some sites can not be rendered like this. Specifically, sites like Twitter (that block attempts to be turned into frames) are the kind of sites that are not compatible with this application.
SitePouch.com In Their Own Words
Group, view and share multiple web pages in one browser window.
Why SitePouch.com It Might Be A Killer
It is a pretty direct way to compare the layout of sites.
Some Questions About SitePouch.com
Which other practical uses has this got? 
