I doubt that the term "one-hit wonders" is a good choice, regarding that most formerly successful but now almost-forgotten programs went through several iterations to become exactly that, successful (not MS-scale, but somehow), and then abandoned.
Regarding Ecco Pro, it was at v. 4 when it was cancelled (in 1997), and you can get it as a free download from NetManage, who own it (http://supportweb.netmanage.com/ts_viewnow/downloads/patchesUnsupported/ecco.asp
). The support page http://supportweb.netmanage.com/ecco/
is still online. There is still a very
active user group at Yahoo (http://groups.yahoo.com/group/eccopro/
), many members run it on Windows 2000 and XP, and some are looking into using it on Vista. There are several other sites dedicated to Ecco Pro. Seemingly it is possible to sync it (basic PIM data) to Palms, but that may break any time. Pocket PCs are another matter, entirely. Ecco Pro had been a big subject on another great site, outliners.com, that just folded recently.
There were some efforts to open-source it , but none were successful (maybe the reason was the database behind it).
I have to say that I took several attempts to really understand that app. I have two thick ring binders full of materials that you can still get online. Great concept. Wish it were still alive.
Having not a lot of experience with the great apps before the all-Microsoft-age, I can only do some kind of software archaeology, but I might throw some other names into the discussion: VisiCalc, Lotus Agenda, Lotus Improv, Lotus Magellan, Lotus 1-2-3, Word Perfect, dbase. X-Tree. Norton Commander. A lot of other things symantec-ed, perhaps (Grandview?).