If you like Notepad++, you'll love Notepad++ Portable. It keeps the same great functionality of its big brother, but in a new edition specially designed to be stored and used in a USB memory key.
Once installed, the application hardly takes 2 MB of space, so it's suitable even for the smallest removable drives. During our tests, we didn't miss anything from good ol' Notepad++: support for dozens of coding languages and several text file formats, syntax highlighting, ability to open multiple documents simultaneously and lots of extra tools.
The only downside to this great little application, if we're to find any, is the fact that you have to download spelling files apart if you want to use the spellcheck.