Unless you’re a web developer or programmer, you most likely don’t really need a whole lot of applications aside from a web browser, perhaps a media player, file manager/viewer and text editor. Maybe that’s why a lot more people nowadays own smartphones, tablets, Chromebooks, etc, and can get away with not using their main computers or laptops for light web browsing. If you wish to have an equally lightweight operating system with just the tools you need but on your actual laptop, you can use Google Chrome OS or Jolicloud.
Today, you can add another name to this list of lightweight computer OSes. Browser Linux is a fast-booting operating system, derived from Puppy Linux, making it a wise choice for any computer, particularly older machines. The most recent version (v. 401, released in May 2011) comes with Mozilla Firefox 4, though you can also upgrade to Firefox 5 once you boot up, or get other versions of the distro with Google Chrome. Like Puppy Linux, Browser Linux can save changes persistently to a USB flash drive with as little as 2GB. The ISO file itself is about 90 MB.