Software you'll need:

GPSMapEdit - The program you'll use to actually create your map
cGPSMapper - The program that creates a file for use with your GPSr
MapUpload - For sending maps to your GPSr

Software you may want:

OziExplorer - Necessary if you're going to be tracing your map using scanned paper maps or other raster maps
GPS Track Maker - Another map creation program that has better tools to edit imported tracklogs. If you're working with tracklogs, you'll probably want this.
Inno Setup - For making Mapsource installable maps.