2. Browse The Web Without Leaving A Trace
A portable Opera browser fits on a thumb drive and leaves nothing behind.
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Another concept that people are at least vaguely aware of is using a standalone browser on a USB drive to surf the Web without leaving traces behind on another computer. This is another scenario where there's more than one way to pull things off, depending on your needs (and your level of ambition).
On the most basic level, you can use a browser in PortableApps, since a copy of Firefox typically comes with a basic install of the suite. For added security you can throw in one of a number of privacy/secrecy plugins, such as FoxyProxy or NoScript.
If Firefox isn't your thing, other browsers are available in USB-key editions. Opera, for instance, has a mobile version in multiple languages, and, as mentioned above, at least one version portable of Google Chrome is being developed.
A variant on the whole PortableApps suite, compiled specifically for the sake of secure browsing, is the DemocraKey. The installation process is the same as setting up a PortableApps instance, and it's functionally similar. What's different is that the installation of Firefox in DemocraKey contains the Torpark proxy-routing modification, so all traffic from the browser can be sent through the TOR network if you so choose.
3. Run A Standalone OS For Dedicated Tasks
This one's also probably familiar to a great many people but it's hard not to mention it here, and it dovetails nicely with the last tip. With a little work you can take an entire operating system -- typically Linux -- mount it on a flash drive, boot it directly, and run it in lieu of anything installed on the PC.
Browsing the Internet without leaving traces behind on the host computer is only one possible application for such things, of course: you can perform rescue operations, do multimedia work... the list goes on.
In theory you can use a Linux distribution of any size, but the upper limit for size is typically going to be the capacity of the flash drive itself. To that end you'll probably want to use distributions specifically designed to run small and light. Damn Small Linux and Webconverger are two of the most commonly used distributions for this sort of thing, but my personal favorite remains Puppy Linux, now in its 4.0 incarnation and better than ever.
When you install an OS to a thumb drive you have two basic setup options, depending on how cautious you are or how much convenience you want. You can either have the operating system boot to memory and run from there exclusively (a la Knoppix) with no persistency between sessions, or install the OS to the thumb drive as a full file system with a slightly greater risk of having the data discovered. The first option means a slightly slower initial boot time while everything is loaded into memory, but the system itself runs quiet snappily after that.
I should note that sometimes security isn't the only reason for having a portable OS. If you're using someone else's computer a great deal, for instance, and you don't want to clutter up the machine with your work/surfing habits, a solution like this is just about perfect.