[jembo]

There's an easy way for you go actually be able to spy on a website and get to know what plugins, apps and themes a certain website is using.

It's not my website but I found this website to be really useful to me for some time now.

Let me know in the comments if you think it is actually useful or not lol

https://www.builtwith.com

[Seagulls]

That's cool. I always used IsItWP to find out if it was a wordpress site. Thinking about it, I think if it was not a WP site that's all it would say. Don't think it could identify if it was shopify etc.

[Dhoroty]

I always checked the source myself, but this site would speed me up a bit :)

[Jam1e]

I believe this site also tells you what analytics software the site is running, what javascript frameworks, and basic information about the server.

Certainly one of my go-to websites when learning more about what my client's old website is using.