Do you have an extra USB flash drive that is lying idle? Why not convert into a multi-purpose rescue tool?

I have loaded my 2GB pen drive with a stand-alone Slax Linux operating system and several portable Windows applications. Installing Slax on USB is as easy as copying a folder, that's what you actually do. The Linux OS provides me the comfort and safety of my own desktop without having to lug around a heavy laptop. Portable Windows applications allow me to use my applications on PCs that lack the application.

Killer portable applications
Portable versions of most popular Windows applications are available on PortableApps and PenDriveApps. Most of these applications are also available for Mac on FreeSmug. Here is a list of some basic portable applications for your USB drive (list for Mac portable apps in a separate post).


Internet
Portable Firefox is a must for Firefox addicts. Its Mac version is also available. Some popular add-ons for Firefox are also available for the portable version. Portable version of Google Chrome should be fine for those who prefer Google's Web browser. Opera lovers should find the right alternative in OperaUSB.

Pidgin Portable, like its regular version, supports instant messaging for several service providers including Gtalk, Yahoo and Facebook.

Skype Portable should come handy for video chats and phone calls.

Though it's not likely that one will start a torrent download on any computer other than their own, µTorrent Portable bittorrent client is there for this purpose.

If you want to carry your inbox on your USB, Portable Thunderbird is the way to go. It is also available for Mac.

For Web developers and tech guys, FileZilla Portable allows downloading and uploading files to FTP servers. PuTTY Portable is a light-weight telnet and SSH client.

Graphics and Images
GIMP Portable allows users to edit images and photographs.

For techies and developers, IcoFX Portable is an application for working with icons or .ico files.

Inkscape Portable is an open source vector graphics editor, an efficient alternative to Illustrator and CorelDraw.

Riot allows you to save images for Web and emails with the best compression ratio and quality. It reduces the file size of images drastically without compromising with quality.

Office utility

Portable OpenOffice Suite is a must for having on your USB drive because some  computers may not have word processor installed. Open Office also comes handy when one wants to create a PDF version of some word file.

Portable AbiWord is a Microsoft Word look alike. If you are hooked to Word, it should be very comfortable to use. It is also very small in size compared with OpenOffice. It's also available for Mac.

Sumatra and Foxit PDF Reader are the two lightweight alternatives for carrying PDF readers of USB drive.

Multimedia
When it comes to playing multimedia files, VLC Media Player is unbeatable. If you like recording music and other stuff, Audacity should find space on your thumb drive.

Antivirus and firewall

ClamWin Portable is the portable version of open source antivirus Clamwin. It doesn't provide live protection but will scan and remove viruses effectively.

Utilities
Occasionally, you will come across computers which do not have an application installed for handling archive formats such as TAR and RAR. 7-Zip will unzip almost all kinds of archive files and folders. It will also compress files and folders and create archives for you.

KeePass Password Safe Portable lets you carry your login information and passwords on your USB drive. You can save all you passwords on it and use a master password to keep them safe.

If you need to synchronize, backup and encrypt files and folders, Toucan comes to your rescue.

Notepad++ is a killer replacement for Windows Notepad. It's a must tool for those who work on codes such as HTML, PHP and C.

Recover Files, as the name suggests, can undelete / recover deleted files. Recuva also enables the user to recover deleted files, including those deleted from flash card and mp3 player.

TrueCrypt is a must portable app for those who carry sensitive data on their USB drives. TrueCrypt can encrypt files and folders as well as create hidden folders, securing your data.

CCleaner is the portable app for cleaning the garbage from your PC. It will delete all the trash left behind by various applications and by users. It also cleans up the registry by deleting obsolete entries left behind by previous applications.

Portable App Suites 
PortableApps Suite includes the basic programs for essential computing needs (Open Office, Fiefox, Thunderbird, Pidgin, etc.) and has an elegant menu system to access the portable apps. You can pick from the various versions of suites available and then add other portable applications according to your needs.

LiberKey Ultimate (186MB) is another power-packed portable app suite that includes 165 applications to meet your requirements. The huge list of portable apps includes almost all kinds of apps from multimedia players to system maintenance tools. LiberKey Standard has 94 apps. LiberKey Basic (56MB) has 26 applications. Once you have downloaded and installed any one of the compilations of LiberKey, you can check the complete list of apps available for LiberKey and add your favourite apps such as VLC Media Player that are missing from the compilations.