KeepNote is a note taking application that works on Windows, Linux, and MacOS X. With KeepNote, you can store your class notes, TODO lists, research notes, journal entries, paper outlines, etc in a simple notebook hierarchy with rich-text formatting, images, and more. Using full-text search, you can retrieve any note for later reference.

KeepNote is designed to be cross-platform (implemented in Python and PyGTK) and stores your notes in simple and easy to manipulate file formats (HTML and XML). Archiving and transferring your notes is as easy as zipping or copying a folder.

Read more about KeepNote's design philosophy or follow my development ideas on twitter (@mattrasmus).
Features

Rich-text formatting (e.g. Bullet point lists, Inline images)
Hierarchical organization for notes
Web links and note-to-note links
Full-text search
Integrated screenshot
File attachments
Spell checking (via gtkspell)
Auto-saving
Built-in backup and restore (archive to zip files)
Extensions (i.e. "plugins")
Cross-platform (Linux, Windows, MacOS X)

Portability

Some people use the term portable to mean that a program can be installed on a removable drive (e.g. USB drive) and that all data is also stored on the drive. This allows someone to move the program and their data to any computer (e.g. between a work and home computer). KeepNote can be used in a portable mode by following these steps:

When installing KeepNote choose the USB drive as the install location. For example, if the drive is called J:, choose a location like J:\KeepNote
Create an empty text file at the location J:\KeepNote\portable.txt
Lastly, make sure to save all your notebooks to the USB drive. For example, your notebook might be saved at J:\MyNotebook1 and another notebook might be saved at J:\MyOtherNotebook2, etc.

Step 2 is needed to ensure that preference data is stored on USB drive. For this example, it will be saved at J:\KeepNote\keepnote. Without this step, user preference information will be saved in the Application Data folder.