One of the biggest issues you face when you first start doing molecular dynamics (MD) simulations is how to create an initial geometry that won’t blow up in the first few time steps. Repulsive forces are very steep if the atoms are too close to each other, and if you are trying to simulate a condensed phase (liquid, solid, or interfacial) system, it can be hard to know how to make a sensible initial structure.
Packmol is a cool program that appears to solve this problem. It creates an initial point for molecular dynamics simulations by packing molecules in defined regions of space. The packing guarantees that short range repulsive interactions do not disrupt the simulations. The great variety of types of spatial constraints that can be attributed to the molecules, or atoms within the molecules, makes it easy to create ordered systems, such as lamellar, spherical or tubular lipid layers. It works with PDB and XYZ files and appears to be available under the GPL. Very, very cool!