I use a simple 2D CAD program called MacDraft to do my layout planning. It is not specialized for model railroading, but I've been using it a long time and it meets my needs.
Here are a couple of resources that you can freely download and distribute for non-commercial purposes:
Bob's Track Planning Questionnaire is the document I use to gather basic information for designing a plan. It might help you organize your own "Givens n' Druthers." It's a fillable Microsoft Word doc.
Bob's Curve and Grade Calculator is an Excel spreadsheet with multiple tabs. It can calculate the length of tracks at various curve radii and grades, along with train lengths and other functions for N, HO, S, and O scales. Have fun.
