Who's Mailing What! is a really simple and useful low-tech idea: scan the direct mail (paper) that's sent to folks around the country. Make this database browseable and searchable to anyone who might want to see, say, exactly how Geico markets its products to consumers. Really useful to other direct marketers.
Why not offer something similar for web pages?
Take screenshots, at regular intervals, of home pages and landing pages of leading web companies.
Not just archive.org (too coarse: Spreedly vs. Recurly vs. Chargify)*. I don't care about links or preserving functionality, I just care about layout, wording, button design, calls-to-action, etc, which have presumably been refined over time by these companies. Most modern web companies respect that home page and landing page design is "a process of ongoing improvement," and employ analytics and A/B testing tools (like unbounce, which is awesome BTW) to track and increase conversion rates.
I suspect that enlightened web marketers (folks who design home pages, landing pages, manage search engine marketing campaigns, display ad campaigns, e-mail marketing campaigns, etc) would pay for a subscription service that showed them how (and when!) their competitors designs have evolved (again, changes to layout, wording, buttons, calls-to-action, etc.).
* P.S. Any way to link to multiple web pages? One link opens n pages? Like how Hipmunk enables you to open multiple tabs on their site?