Simon 5 is coming soon!
Simon version 5 is currently in beta testing. It is a massive update, introducing a much-requested feature: syncing the Simon data between multiple Macs, plus a Simon Status service, a Link Checker filter, improved Find filter, iMessage and Speak Error notifiers, a new app icon, and much more.
Important: please note that Simon 5 requires a minimum of macOS 10.12 (Sierra), and will be a paid upgrade from Simon 4 after the beta. Purchasers since September 1, 2020 automatically get a version 5 license (that also works in previous versions).
Re: Growl Notifications between 2 Networked Computers
I can't really answer the Growl question. I vaguely recall seeing a remote Growl notifier somewhere, but I might have imagined it.
I do plan to enhance Simon in the future to enable sharing data between multiple machines, which would enable notifying via Growl, sound, speech, or whatever on a different machine than the test is performed (and checking on multiple machines). But that's in the future.
Re 2.6, I am currently wrapping up my secret new iPhone app... then I'll be working on an iPad edition of it in March. Once that's done, I'll be moving to Simon 2.6, in April. I'm looking forward to getting back to Mac development!
(Oh, and there will be Simon 2.5.6 next week, too; just a quick bug-fix update.)