Someone has. Osalt.com -- "Open Source as alternatives" -- lets you browse open source software in a variety of categories.
The directory includes business, communications, databases, graphics, networking, security & privacy, system utilities, and Web development. Within each category, you'll find a slew of commercial apps and open source alternatives listed with sort descriptions and links to the publisher's Web site.
This isn't the only online resource for open source alternatives -- we've previously told you about AlternativeTo, for example. But Osalt listed options I haven't run across before.
On the downside, you can't get a list of alternatives by selecting a specific commercial app like Word or Photoshop -- you're limited just to browsing by general categories like business or graphics. But you can filter the results to show just Windows, Mac, Linux, commercial, or open source if you prefer.