Until recently, full solution of the complex set of equations that describe gas-particle flows was nearly impossible. But an open-source code developed by researchers at the National Energy Technology Laboratory (NETL), and designed to run on inexpensive PC (Beowulf) clusters, makes it relatively simple. Called MFIX (Multi-phase Flow with Interphase eXchange), the code incorporates special numerical techniques that provide an efficient solution to the coupled equations, exceeding the capabilities of commercial software. Originally developed at NETL to model fixed, fluidized and bubbling coal gasification technologies, its power has been used in research ranging from catalytic cracking in oil refineries to volcanology. A free, fully-functional version is available at www.mfix.org