Looking for an alternative to a program for Windows, Mac, or Linux, or a substitute for a web app? Try AlternativeTo.net. Just type in the name of the software you'd like to see alternatives to, and you'll get a list of alternatives ranked by popularity.
Let's say you're looking for alternatives to Dropbox, a popular cloud (online) storage program. Just search for "dropbox", and AlternativeTo offers a list of other web-based sync and storage programs, including SpiderOak, Microsoft Live Mesh, Mozy Home, Windows Live SkyDrive, and Box.net.
Looking for a cheap or free alternative to PhotoShop? AlternativeTo gives a few choices: GIMP, Paint.NET, Pixlr, and more. Here's how GIMP appears in search results:
You can sort search results by the number of "likes", filter by license (commercial, free & open source, open source), and filter by platform (Mac, Windows, Linux, Online), so you can find exactly what you're looking for.