My school computers don't allow installation of software in a limited account, but I simply couln't bear using Internet Explorer. After some Googling, I found a portable version of Firefox, perfect for my use.

This list of USB-friendly apps is inspired by that.
Portable Firefox:
Available at: http://johnhaller.com/jh/mozilla/portable_firefox/

The free and safer browser alternative to internet explorer. A fast and easy to use browser with hundreds of plugins to customize Firefox to your liking. It also features tab browsing and pop blocking built in.

Portable Thunderbird:
Available at: http://johnhaller.com/jh/mozilla/portable_thunderbird/

Mozilla's email client that can handle several email accounts individually. Thunderbird also provides effective tools for detecting junk mail.

Portable Sunbird:
Available at: http://johnhaller.com/jh/mozilla/portable_sunbird/

Sunbird is a calendar program. Intended for the user who uses Mozilla Firefox and Mozilla Thunderbird and wants a calendar application based on Mozilla.

Xmplay 32:
Available at: http://www.un4seen.com/

XMPlay is an audio player, supporting the OGG / MP3 / MP2 / MP1 / WMA / WAV / CDA / MO3 / IT / XM / S3M / MTM / MOD / UMX audio formats, and PLS / M3U / ASX playlists. A load more formats are also supported via plugins.

XnView:
Available at: http://www.xnview.com/

XnView is a utility for viewing and converting graphic files. It can import about 400 graphic file formats and Export about 50 graphic file formats. It can also Copy/Cut/Crop and resize graphics and add effects to them.

Foxit PDF Reader:
Available at: http://www.foxitsoftware.com

Foxit PDF Reader is the only eligible alternative reader/viewer for PDF files (eBooks). It's smaller, faster, and cleaner. And it's free.

AEdit - Free Word 4.0 SX:
Available at: http://web.tiscali.it/axart/english.html

A Free Wordprocessor with strong encryption, as comfortable as possible. This is an essential version, optimized for speed.

i.Ftp:
Available at: http://www.memecode.com/iftp.php

i.Ftp is a little graphical FTP client, which does what you need in an FTP client without the size and fuss.

HJ-Zip:
Available at: http://freebyte.com/hjzip/

HJ-Zip is a simple and easy to use zip/unzip program with full Windows user-interface. You can zip and unzip files, create zip archives, and also make self.extracting archives.

Cdex:
Available at: http://cdexos.sourceforge.net/

Cdex is a tool to do all sorts of things audio related. Mainly focused on ripping and converting, things like turning your home Compact Disc collection into an mp3 collection on your hard drive become extremely easy. With built in support for many encoders you wont find any shortage of options for your media files.

I like the portable Firefox one, very handy.

If I am at someone else's house but still needs internet access, I use Portable Firefox from my thumb drive.

Some great links there chiawaikian. That's a cool idea about the portable Fx as well! 8)

a stripped-down browser ... http://www.offbyone.com/

an irc chat client ... http://www.tinyirc.net/

both run without "installing". so they can run off a floppy, usb, cd, or harddrive.

Portable trillian is great I have been using thoose for months