I’ve used hundreds of different programs in the 20+ years I’ve owned a Mac (I’ve got 15 running right now), but the two I’ve used most consistently, for more than a decade have been the word processor NisusWriter and the email package Eudora. I’ve just downloaded the last commercial version of Eudora for the Mac (6.2.4, the Windows version went to 7.1.
The pill has been sweetened by the announcement that the Eudora code will be released as open source, but I can’t see anyone stepping forward to work on this, at least for the Mac, given that Mail is freely bundled with OS X and there are several excellent free or low-cost alternatives.
Of course, the program still works, so there’s no need to change it any time soon. And Nisus made a successful transition to OS X quite a while ago, with the new name Nisus Writer Express.