USB 'pen drives' have totally changed the way we can carry our documents and media around with us. For a few quid we can pick up a drive that will store hundreds of our important documents, or our favourite pictures or music and video files. Carrying our data around is brilliant, but there's no guarantee that the computer you're taking the drive to has the software to actually be able to view or manipulate the data. That's where Portable Apps comes in.
When you install an application to your computer, files can be placed all over the place. It generally isn't possible to simply navigate to the 'Program Files' folder and copy your application to a USB drive, as when you ran the installer for that program, it will probably have placed all kinds of files into Windows system folders and made several entries in a data store known as the 'registry'. A portable app, on the other hand, is an application that stores ALL the data it needs in the same folder. This means that it can be installed on a USB hard disk or, even better, on a USB Pen drive, and executed from any computer that runs the Windows operating system.
portableapps.com is a website which hosts an amazing array of free, (mostly)open source, portable apps. Popular web browsers such as Firefox and Google Chrome can be installed on your pen drive, meaning you can take your browser, including bookmarks and your favourite extensions, with you. There are also several instant messaging clients such as aMsn, and even a full version of Skype to keep in contact with your pals.
There's a full, free, office suite - complete with word processor and spreadsheet - which is compatible with Microsoft Office file formats.
Graphics nuts will be pleased to be able to carry around GIMP and Inkscape in their pockets, while developers are well served with a variety of text editors, integrated development environments, and server & database management software.
Of course, not every field is so well catered for. Portable apps are of course reliant on open source software, or software houses releasing portable version of their programs. To that end there is very little in the way of audio/midi sequencing software, or video editing tools - though playback and file format conversion is well catered for. A sequencer and VST instrument host would be most welcomed by the music community, I would imagine!
All in all, there's nothing you can really say against the concept of portableapps.com - free software that you can put on your USB drive and run anywhere, giving you the ability to manipulate your data in the way you want to.