AlternativeTo - AlternativeTo lets you find apps and software for Windows, Mac, Linux, iPhone, iPad, Android, Android Tablets, Web Apps, Online, Windows Tablets and more by recommending alternatives to apps you already know.
Softpedia - A library of over 1,000,000 free and free-to-try applications for Windows, Mac, Linux and Smartphones, Games and Drivers plus tech-focused news and reviews
Opensource Builders - Find open-source alternatives to commercial apps
Product Hunt - A website that lets users share and discover new products
SoftwareSuggest - SoftwareSuggest.com is a business software discovery and recommendation platform.
Filehippo - Filehippo is your key to opening files that Windows claims are "in use by another user" or simply "can't be found." Avoid the hassle.