It helps me find the technology behind particular websites
I go to BuiltWith whenever I need a lists of tools that dominate the particular category of software solutions. I also use it to lookup the websites that use certain web technologies. I like the market share information and country based analytics and filters. Database seems to be updated quite often.
Using it for lead generation is somewhat controversial, because it ends up with outbound sales approach to people who never requested it. It's quite expensive too.
In-depth information on today’s web development trends
BuiltWith is a robust application that provides transparency to today’s technology. This stands to revolutionize the way we do business today as experts gain insight into the applications being used to power the world’s best websites.
Provides in-depth information today's web development trends
Price could be out of reach for many businesses
Great Research Tool for Scoping Website Projects
I use this browser extension almost every day, certainly each time I'm researching a prospect or competitor website. With one click you learn about the technology running on the target website. It's great to be able to quickly tell a prospect "Oh I see you're using wordpress with the Avada theme and there's a Facebook pixel so I'm guessing you run FB ads." It's impressive to the non-techy client and helpful in getting your bearings on a new client. I haven't used much of the options on their website, I just stick to quick glances on the browser extension.
Some websites block this app from reading the build. It doesn't offer whois lookups, which I think would be a good feature for this quick scan. Some features are paid.