Portable Security

Truecrypt is great, but it's not very useful for this purpose because it
requires installation on the target PC, which in turn needs admin rights (it
installs a device driver). It's OS independent though so you can get to your
files from both Windows and Linux PCs (for Linux they have a command line
tool, although there's a GUI in development for Ubuntu/Debian/maybe other
distributions). Not sure about Mac support.

The only program that I've found that sort of works is Rohos Mini.
In that version, it's free, but the size of the container files is limited
to 300 MB. You can create multiple containers though.
Initially, you have to install the software on your computer, and when you
create a container on your USB stick, it copies additional software onto the
drive that'll let you access your encrypted data on any (Windows) computer
without having to install anything. I don't think you can add data to the
enctrypted drive though, unless you install the program, which, of course,
needs admin. rights.
Hm, actually, maybe this [read-only access everywhere] will work with TC as
well, haven't tried it yet.

Quite a few USB sticks come with their own encryption software, but those
tend to work on the file level. And every file/folder you want to access has
to be decrypted first - either on your USB stick, or on the hd of the
computer you are connected to.

Then of course there are commecial solutions. Some are quite interesting,
like the IronKey device.