Toontrack is famous for its November sales, where you can often find significant discounts on the core software and SDX expansions.
Toontrack occasionally releases free and Sound Drops (individual preset snaps). While not the Core Library, you can download their free "SD3 Demo Project" WAV stems to see how the raw drums sound in a mix.
This is widely considered the best free drum plugin. It includes a fully functional kit from the SSD5.5 library with no time expiration.
If you want the ecosystem for free, look at EZDrummer 3. It is the little brother of SD3.