MyTetra (Version: 1.30 tested) is an open source and cross platform note taker, notes organizer or PIM. It doesn’t have the most modern interface but it is a good option if looking for an organizer.
MyTetra is tree structured. Its structure is item (or sub item) with separate note(s) within each item or sub item (see image). It comes with a base item and sub items or siblings or other items can be added. MyTetra has an unusual characteristic, it requires each item to be divided into notes, the nodes in the tree themselves don’t hold any text, just a list of notes that belong to them. Notes are added by clicking the Add Note (top middle of interface) button.
Note content appear below the notes list. The note editor includes paragraph, justification, font, indent, list, table and image tools. Additionally there is an Edit HTML button to edit or work in pure HTML.
Search is a strength of MyTetra. One can search within an open note, if necessary, by clicking the Find button to the right of the font selection dropdown but more usefully the find section occupying the lower part of the program can search across all notes and all fields (tag, author, url, title or content or text). If the find area is not visible it can be toggled on at Tools—>Find In Base. Tags belonging to an open note are visible under the note area and are clickable to aid in searching for other notes with the same tag.
MyTetra supports copying and pasting nodes so one can create duplicates. It also supports encryption (Tools —>Settings —>Crypto) and Synchronization over a version control system like Git (Tools —>Settings —>Synchro). It is available for Linux, Windows, Mac OS X and FreeBSD.