I was prompted to post this when another thread described the HP35 as "science fiction" to its contemporary users.
As someone who grew up with pocket calculators (just), I tend to take them for granted. However, I've recently bought a copy of Derive for DOS and installed it on my HP200LX. The 100/200 already has a good calculator implementation, being pretty much a 17BII clone +/- some bits & pieces, but Derive lets me know how the early 35 users must have felt. It's awesome!
To describe what Derive can do would take pages. If you don't already know and are curious, take a look at http://www.chartwellyorke.com/dfdmajor.html for a brief rundown.
BUT...is it an HP calc? If not, then what about a 48 or 49 running an externally written application? For those who don't know, the TI89 & 92 run a scaled-down version of Derive. So here's an HP answer to those calcs. Except, of course, that HP don't make the 200 any more. Sounds familiar!