Slitaz: home brewed since 2007
Many of the lightweight Linux distros on offer are based on more popular desktop variants such as Debian, but this one's grown completely from scratch since 2007. It's one of the few that includes languages other than English (Spanish, French, German and Portuguese).
The base install is competent enough for a variety of tasks. The browser is Firefox 3.5, which may not be the most lightweight app you could think of installing, but it does give Slitaz the ability to run pretty much any web app, which is what many people will want to do with such a diminutive distro that doesn't have a lot of its own software.
That said, there's a cluster of useful tools included as part of the minimal install, including a MTPaint, a PDF reader, music player and a couple of editors (Leafpad and Nano). For lightweight and embedded projects, it rather unbelievably includes a fully functional webserver (Lighttpd) with PHP/CGI support, and various other standard network tools as well (such as SSH and FTP).
If you feel the need to bloat out the system, there are over a thousand packages available in the online repository. Package management is via a tool called Tazpkg, which is tiny, but straightforward and easy to use.
The packages themselves are custom archives with included information and dependencies, so you won't get caught up in a whole world of install pain (though you are limited to the packages available from the Slitaz repository, unless you want to make your own).
The desktop uses the nippy but low overhead Openbox window manager, combined with LXDE desktop, which should be pretty intuitive to most users (it's most akin to a KDE 3.x desktop).
Slitaz achieves the objective of cramming a lot into a small space. It doesn't have an overwhelming selection of default packages, but they do the job, and they do it very fast.
Version: 2.0 Cooking
Exceptionally quick, deceptively powerful and has a built-in webserver
Tiny Core Linux: smaller than the smallest thing
The Tiny Core project was started in 2008 by one of the refugees from DSL, so it isn't much of a surprise that it follows the same ethos of trying to get as much as possible into the minimum amount of space.
If anything, Tiny Core has taken this to more of an extreme, completely savaging the package base to create just about the smallest distribution you could still consider to be a Linux OS. While this is great news for those trying to fit the OS on to ancient hardware or embedded devices, it does inevitably mean you'll need to do more work if you want to do anything other than boot it up and look at the X display.
Fortunately, there's an app installer that enables access to the large repository of TCZ packages, so you can easily install the apps that you want. Dependencies are handled, but obviously, if you choose to install something like Firefox, you're going to see the disk space taken up by this distro ballooning to new levels. But you will have to install something, otherwise a few system scripts and a terminal will be your only company.
In some ways, it's not quite so useful to have such a diminutive distro. There may be some specialist cases, but for general use, most people can easily spare, say, 100MB of space. Sure, you can build on the Tiny Core install by adding applications, but it may have made things easier to aim for a slightly higher target to begin with.
But that's to take nothing away from the remarkable achievement of creating a Linux install that fits inside 10MB of space. It's easy to see Tiny Core becoming the basis of many specialist application distros – if you can get the base install down in size, it leaves you with a lot more room to pile on your custom applications.
Verdict: Tiny Core Linux
A remarkable achievement, but requires effort to install and use