ithought is a personal thought manager, allowing you to create and store entries that can be linked together via keywords. Through the use of various modules, it is possible to do a number of things with these entries, including uploading to a website as part of an online journal.

MindMapping displays knowledge in a tree structure so that the associations and links between knowledge "nodes" are visible. This implementation additionally allows one to archive the knowledge in graph form instead of a tree structure, to have multiple roots, and to add images, databases, and translations of whole books

Think is a Gnomified outliner.

ThoughtStream is a program that allows you to store information and organize it the way your mind works -- by associations or links between ideas.

ThoughtTracker is a knowledge base application of sorts. Its purpose is to provide you with an efficiently-interfaced database in which to store those infrequent ideas that are worth keeping or facts learned by experience you don't want to forget but don't know where to put.

ToutDoux is a project manager which lets you design a plan of action using a tree structure, with translations in French and English.

yank is yet another notekeeper for gnome.