How can SaaS Freemium be defined:

Give the service away for, possibly ad-supported but maybe not, acquire a lot of customers very efficiently through word of mouth, referral networks, organic search marketing, etc, then offer premium-priced value-added services or an enhanced version of your service to your customer base.

Examples of Freemium SaaS products:

1) Skype
2) Box
3) Gmail
4) YouTube
5) Evernote
6) Notion
7) Mailchimp