As the name would indicate, Ear Trainer is designed to train people's ears.
Specifically, for intervals and harmony. There is a panel for each, as well
as a mini-keyboard. Each panel will play a sound (interval or harmony) and
you answer. It keeps score for that session, but the score isn't important,
it's a tool to help you recognize the weaknesses in your ear, and learn what
to work on.
The keyboard is also fun, it is small but it uses the computer keyboard layout
pretty well, and is the only one I've seen to use the spacebar as a damper.
Ear Trainer is available here.
Noli n Nali Barcode Generator
Not a commercial app, and no longer useful since the company went bankrupt, but I
still love this one. It generated PostScript™ barcodes which I put on the
labels I designed. I learned PostScript™ long ago, when I was part of a team
working on PS & PCL integration. Barcodes (well, the current ones, anyway) are
fairly simple as they are just vertical lines with different strokes. Once the PS
was written to a file, it was a simple matter to import it into Illustrator.
PostScript™ turns out to be the basis for html 5 canvas drawing, so it's
still a very useful thing to know.
Pretty simple widget. Acts as a stopwatch, has 5 lap times.
Stopwatch is available here.