Varnish Cache is extendable by the way of Varnish Modules (VMODs). They are imported and called from you VCL to offer a larger set of capabilities to your setup.
The following list contains the available VMODs we package and support in Varnish Enterprise:
Besides the VMODs presented here, there are many open source VMODs.
An ongoing effort to describe them can be found at https://varnish-cache.org/vmods/.
See also https://info.varnish-software.com/blog/varnish-modules-vmods-overview.