Portable applications can be run right from a USB drive such as a thumb drive or even an iPod. Unfortunately, not all applications can be run from a USB drive. Here’s a few great resources for portable applications. As far as I know, all the applications listed on these websites are freeware.
This website offers a great selection of software that has been optimized to run from a USB drive. This is probably the premier place to find portable applications. At this point in time, PortableApps has 40 applications listed on their website. Another great thing about PortableApps is that they have their own suite that you can download which includes a fancy “PortableApps Menu” which acts more or less like a start menu for all the programs on your drive.
This resource has a large amount of software available and has over 1,000 applications available currently. You can find a lot of software here that you may not have even heard about before. Portable Freeware Collection is a great place to find great little tools to put in your “tune-up kit” when you go over to help grandma with her computer.
This blog links to a bunch of great portable software that has been created by others. Not all the applications on Pen Drive Apps are freeware. Here you may be able to find a portable version of a program you have paid for, which can come in handy if the developer doesn’t offer a portable version. Personally, this is probably the last place I’d go for portable applications.