How to monetize a keyword research tool?

Hello !

My & my friend are building a web based keyword research tool. It will be a free tool and no registration required to use.

We don't have plans to force users to join the site & pay and use the tool.

Since it is a free tool, how we can monetize it ?

1. Selling ad spaces
2. Having a opt-in pop-up and build list
3. Promoting affiliate offers by display banners

This is what we have in mind for now. any other creative ideas?

Thanks