Fuzzy numbers and basic fuzzy arithmetics (+, -, *, /, 1/x) implementation written in Java. Should be used as a library, but it also has a GUI which demonstrates its capabilities (using SWT widgets).
Triple-A (Automatic Analysis with Automata) is a full-featured library for (finite) automata written in Java with two powerful analysis tools (dealing with LTL model checking and Presburger arithmetics) and a graphical user interface built around it.