One, two, three and four --the talking practice metronome that speaks the beats out loud.
***** great metronome app
Very precise and versatile: lets you control the time signature, select quarter notes, triplets, etc, and even which specific beats she says. This is the only metronome app I use now.
 ***** Great interface on this metronome!
Because this metronome counts out loud in english it is particularly useful for practice. I look forward to using it when learning new dance steps and dances as a result!
And the interface easily supports tempo changes on the fly!

3 in 1 Improviser

Improvise! Compose! Conduct! No Experience Required 
*****  Great demonstration of Spontaneity.
I saw Gary Marcus' demonstration of this app in front of Pat Metheny, at the "Music and the Spark of Spontaneity', one of the World Science Festival events at the Cooper Union last night. This app does simulate what I have been thinking about the way in which how improviser's brain selects each note in sequence...

***** Fun improvisation tool!
 ... I can already tell that I will be able to entertain myself for hours with this app!