xfig

Notable because it is easily the most popular open sourced object-based drawing program I know of, and because it provides me with the ability to do several things, all of which are related to school at this point:

• Draw graphs for my Combinatorics and Graph Theory homework, complete with arrowheaded edges and nice, compact vertices.
• Draw finite automata (finite state machines) for my Introduction to Computation Theory homework (also state diagrams for Turing machines).
• Export the drawings to PostScript or Encapsulated PostScript format for inclusion into TeX and LaTeX documents (my tool of choice for homework these days).