GnoTime: Keep your projects under "timing control".
TODO list / diary / journal tool
Gournal, Note-taking tablet application. A paper-like interface allows you to hand-write notes. This is a gnome app (actually, gtk2/perl).
Jarnal, a Java-based note-taking tablet app.
QBall's GToDo is a simple Gnome2 to-do list application.
Quozl's GFocusTimer tracks which windows your mouse is in.
GTimer, an older gtk-1.2 timer application.
PTT another KDE-based time tracker.
Titrax is a very old (early/mid 90's) X-Windows time tracking tool. This is a nice historical snapshot providing a glimpse of the way things used to be in the Unix world back then.
LinuxLinks provides a huge list of Linux timer software.
Journyx, an up-scale, commercial web-based time tracking and billing tool.