alternativeto.net has a better database and a better interface, imo. You search for the software you want an alternative to, then choose your platform and license, and get a list of alternatives sorted by popularity. For instance, look up Altium (electronic schematic/PCB CAD package), and it shows results like KiCAD (open source, free), Eagle (very proprietary), LibrePCB (new open source project), Schematics.com (web-based). It even has entries for program features and description, so you can see what makes a program or service an alternative.