Since it's income tax season here in the USA, let me shout out "Tax Preparer" by HowardSoft. First developed by Jim Howard in 1979 for the Apple II, "Tax Preparer" was featured on a 1985 "Computer Chronicles" episode, with Howard appearing in-studio with Stewart Cheifet and Gary Kildall.
41 years later, Howard is *still* updating and releasing "Tax Preparer" for the PC. It's almost certainly the oldest continuously developed program featured on "Computer Chronicles."
