SPLAT! (short for an RF Signal Propagation, Loss, And Terrain analysis tool) is a GNU GPL-licensed terrestrial Radio propagation model application initially written for Linux but has since been ported for Windows and OS X. SPLAT! can use the Longley-Rice path loss and coverage prediction using the Irregular Terrain Model to predict the behaviour and reliability of radio links, and to predict path loss.
There is a similar application to SPLAT! called Radio Mobile which is also free for amateur radio operator use.