PortableApps is very convenient application management system which allows you to carry your favorite applications on USB drive and can be downloaded at PortableApps.com.

Not all applications can be made portable because of

• How application interacts with Operating system
• If application is not free. 
• Not all companies allow free distribution of their software.

However, PortableApps currently offers strong enough application suite for daily need. Check it out!

Following applications are not distributed by PortableApps but can be put on portable drives. Great part is that most of the applications can be just unzipped/installed into portable application folder and PortableApps menu can still provide access to these applications. Currently I have tinkered with following applications (outside PortableApps)

• Mercurial - Distributed Revision Control System
• Foxit Reader - Great Adobe PDF reader replacement
• uTorrent - Bit Torrent Client
• wosPortable - Portable Web server, use mediaWiki for your personal documentation
• XeroBank Browser - Anonymous Browsing
• Skype - Intenet Telephony
• TrueCrypt - Secure encrypted filesystem
  

Biggest advantage is to carry these applications on USB is that you do not have to install any application on public computers. Also browsing using portable firefox/xB on public computer is safer as your personal information stays on USB drive.