BFlow is a function library for doing computational fluid dynamics modeling in Excel. It allows users to create Excel models that will calculate fluid flow, heat transfer, and mass transfer in simple 2d geometries. It is a work in progress, but the basic functionality is there. Its a great way to experiment with modeling. Check it out. Its free.
FlexPDE is a great general purpose program developed by PDE Solutions for numerically solving partial differential equations. I have developed several models with this tool and they are posted here. These models are a bit old now, having been developed with version 2 of the code. Still they demonstrate the capabilities of the tool.