Tomboy is a desktop note-taking application for Linux, Unix, Windows, and Mac OS X.
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Zim brings the concept of a wiki to your desktop.
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PNotes is light-weight, flexible, skinnable manager of virtual notes on your desktop.
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KeepNote is a note taking application that works on Windows, Linux, and MacOS X.
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Nevernote is an open source clone of Evernote designed to run on Linux.
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NoteLab brings the power of digital note taking to Linux, Windows, Mac OS X, and Solaris.
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Xournal is an application for notetaking, sketching, keeping a journal using a stylus.
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