I have an 83 Plus.
I've made lots of programs on it. Just to name a few:
A 'speak' program - a silly little program that said phrases that meant nothing, however with correct grammar
Several programs to help out with math - my most complex and useful would be the one that calculated expected outcomes from actual outcomes for matrices, for stats
A factor finder - Input a number, and it calculates all the factor pairs for that number. I never had a use for it, but I think I gave the code to another blocklander who needed it.
A path follower - Draw a path in a certain format, input the starting and ending positions, and it will follow it. Kind of like a maze solver, except it only follows one path, no splits, turns, dead ends, or that stuff that would make it a maze.
Several things with falling characters, including * to be snow, etc.
there's not really that much of a difference between the 83 and 84
just a liiiiiiiiiiiiiiitle bit more ram
It's also noticabley faster. One of the programs I made was a timer, accurate to about half a second per minute. When I sent it to my friend's 84, I had to slow down the loop quite a bit to get it to work accurately.