Xholon is a flexible tool for multi-paradigm (UML 2, ABM, SBML, NN, GP, PSys, CA, etc.) modeling, simulation, design, execution, and transformation. It is based on generic Java and XML building blocks, and optionally packaged as an Eclipse plugin for UML/SysML/FSM simulation. The Xholon project explores the idea of software as systems of linked nodes, organized hierarchically. The Xholon toolkit supports this back-to-basics approach, and demonstrates practical benefits through examples from numerous domains.