Have u checked my previous post  Worlds smallest linux computer and linux networking server ?

After seeing the popularity of that post, i thought it would be good to inform you about worlds smallest Operating system too. Being a great Linux fan, i always support Linux.I am glad to inform you that the smallest OS is a linux based  OS.Its actually not a linux, but its a Linux variant.

It is known as KolibriOS.  KolibriOS (also known as KOS and Kolibri) is a free operating system with a monolithic preemptive, real-time kernel, video drivers, for 32-bit x86 architecture computers, developed and maintained by The KolibriOS Project Team.Its only 2.4 MB in size.  KolibriOS is a derivative of MenuetOS, written entirely in FASM (assembly language). However, C–, C, C++, Free Pascal, Forth, among other high-level languages and compilers, can also be used in user application development. KolibriOS is available in English, German and Russian. It is named after the German and Russian word for hummingbird – Kolibri.

Screenshots (click on images for a Larger Version) :

Latest Stable Release : KolibriOS 0.7.7.0

ISO Size when Zipped : 2.8 MB
ISO size when unzipped : 4.36 MB

Some of the worth mentioning  Features of this OS  are :

1. Boots from several devices.

2. Fits on a single 1.44MB floppy (many applications are compressed).

3. CD and DVD support.

4. USB support (uhci controller).

5. User can change themes directly in the OS.

Try this OS at your Leisure times , u will find it very comfortable and reliable.Also this is an ongoing Project, so please feel free to support them by reporting bugs  :)

Also u may try Tinycore Linux too which is only 10 MB in size but a little more stable than Kolibri.It gives you quickest access to an OS and fastest wired internet connectivity.