I have 16GB of system RAM. Currently, about 2GB is in use (1GB by the system + 1GB by ZFS). I feel like it's a waste to leave so much RAM unused. Are there any good ways to utilize the free RAM?
Enable more services that use RAM. Some of the worst offenders in RAM usage is Squid.
Enabling all rulesets with os-opnproxy almost requires 16GB RAM or the load fails.
Reserving space for sessions ("Firewall: Settings: Advanced" -> "Miscellaneous" -> "Firewall Maximum States") (hit the info button on that one; modifying it is not usually beneficial) or aliases ("Firewall: Settings: Advanced" -> "Miscellaneous" -> "Firewall Maximum Table Entries") are possibilities. For aliases, have a look at "Firewall: Aliases", at the usage bar directly to the right of the title. GeoIP (e.g.) will eat a very large amount of RAM.
You can also eat a bunch at "System: Settings: Miscellaneous" -> "Disk / Memory Settings (reboot to apply changes)". These are somewhat specialized, and should be applied only as needed, with care.