Riktig is likely the best example of everything built on Stact. Cashbox is written on an older version of Stact and isn't a very big codebase, so might be a place to start.
Stact mostly exists so we can build the other cool stuff. Akka.net might be a better solution if you want full blown actors - though I've never used it. They are more interested in supporting an ecosystem around the library than Stact has gathered to this point. Unless they are hidden from me, which is possible.