A persistent USB stick provides you with the equivalent of a Kubuntu computer that you can carry around in your pocket. Any files you save or changes you make will be saved on the USB stick too.
I’m experimenting with different software to control RepRap after having limited success with the RepRap Host Software. The various RepRap software packages seem to work better on Linux than Windows:
The Host software is still a little buggy on Windows…
http://reprap.org/wiki/Installing_RepRap_on_your_computer
so I have decided that running my next choice, ReplicatorG, on Linux might be worth a shot. However I don’t want to have a dedicated Linux computer, so a persistent USB stick seems to be the ideal solution for me 
There are instructions for creating a Portable Installation of the RepRap Host on the wiki using the Ubuntu distribution of Linux. It is essentially the same process as for Kubuntu, which goes something like this:
Prerequisites
- Windows XP/Vista/7 PC able to boot from USB
- USB Stick at least 2GB in size
- Kubuntu ISO
- Universal USB Installer
Creating the Persistent Stick
- Insert your USB stick into a spare USB port, making a note of the drive letter assigned to it.
- Run the Universal USB Installer in administrator mode.
- Select the Linux Distribution, in this case Kubuntu 10.04.x/10.10 Desktop i386/amd64.
- Browse to the Kubuntu ISO file you downloaded.
- Select the drive letter corresponding to your USB stick.
- Format your USB stick as Fat32 if this has not already been done.
- Choose persistent file size – this is usually about half of the capacity of your USB stick.
- Hit Create
- Wait for the Universal USB Installer to complete – this took 10-15 minutes for Kubuntu with 2GB persistence on my Windows 7 netbook.
- Reboot your machine.
- If your PC does not automatically boot from the USB stick into Kubuntu you will need to change your boot sequence.
- Choose “Run Kubuntu from USB stick”.
All being well, Kubuntu should load. Now time to explore!
Update
The stick works great on my Windows 7 netbook, but fails to boot properly on my Windows 7 desktop PC, probably because it is 64bit. So beware!