Dia "A Gtk+-based diagram creation program"; supports UML.
GIMP has a "gfig" plugin for simple line drawing.
ivtools SW library for such tools; from Vectaport; XFree-style licence.
TCM - Toolkit for Conceptual Modeling For diagramming. "a collection of software tools to
present conceptual models of software systems in the form of diagrams, tables,
trees, and the like." Esp. good for requirements engineering.