LiberKey is a free software program which allows you to install and support more than 250 open source portable applications with just one click. No kidding its the ultimate Swiss army knife for the open sources software like Pidgin, USBDeview, Audiacity, utorrent etc.
You can install it your USB device and since it comes with a categorized menu like graphics, download, ftp etc you will find it pretty easy to find your applications. It also takes care of keeping your software updated when it comes.
Also one more thing you would love about Liberkey is how beautifully it allows you to change the application association. So if you would like to open JPEG image with Paint.Net you can do it.
I have an old pen drive which is like 512MB and the Ultimate Pack of Liberkey will be a perfect home for that. You might be a big fan of MAC or Windows Or Linux but remember it never hurts to have these portable apps with you. They are big time handy when you need an alternative. Check out LiberKey