While there might not be direct alternatives to Army Builder that offer the exact same features, there are other tools and apps designed for army building and list management for WFB and 40K. Some of these might be free or open-source.
A powerful, web-based list builder that is compatible with Battlescribe files. It is fast, free, and works on any device. While there might not be direct alternatives to
Fan-made balancing projects that kept old editions alive after Games Workshop discontinued the Old World setting. Classic Warhammer 40,000 Files While there might not be direct alternatives to