Examples of free web apps as lead magnets?

Hi all,

I'm looking for good examples of free web apps or tools as lead magnets for a paid product.

I've been running one such free tool and it is quite popular (notioninvoice.com/datepop), and looking at driving more traffic from it to my paid product (notioninvoice.com). So I'm looking for some good examples that I can learn from.

Have you run a successful lead magnet? Know of any good examples out there? Any leads (ahem) are appreciated!

Thanks