I use XTrkCAD, and am very happy with it for the small layouts I like to experiment with. It has an extensive N scale Unitrack library, other sectional track products, as well as facilities for designing with flex and hand-laid trackage. The price is certainly right ($free), which keeps more budget available for other RR acquisitions. There is an interface to export to JMRI Panel Pro too, but I have not tried that yet. I have found the development team receptive to thoughtful suggestions for improvements and additional features.