Split your browser window in two! Add any site you like as a sidebar to other sites.
Merge sites you frequently use together and visit them at the same time!
Some of the popular sites like Facebook or Twitter use an iframe inside their sites, so you won't be able to use them as a split on a default configuration because of Content Security:
If you really want to, you can disable this in Chrome by running it with the --disable-web-security flag or by changing the security.fileuri.strict_origin_policy setting on Firefox