As with all programs, there is possibility for abuse in this. Please use with caution. Otherwise I'll come after you with a bludgeon.
I got down to writing a calculator virus one day. It actually took several months, because I had other things to do. I made several versions. It's named after Ovid, because I needed a four-letter word (kind of for padding) and my Latin class was translating Ovid that semester.
Apparently this is now featured on the Symantec Security Response Weblog. Hooray? It was subsequently picked up on by Wired's blog Threat Level.
Symantec calls this code TIOS.Divo
.
After a quick websearch, however, it appears that Piotr Bania wrote another
virus, and that is what the Symantec page is talking about. Hooray
for lack of hypertext. His code is interesting; it moves the TI-OS
JUMP_TABLE and hooks into SymFindNext rather
than doing the infection in the virus itself. He took his code
offline, unfortunately.
I made a few variants of Ovid: