Undeniably, iPhone is the hottest cell phone in the market nowadays. I personally used an iPhone too.
I love it because it is not only a very productive tool, it also allows me to carry the piano with me all the time.
Oh.. of course it is not a full 88-keys piano! I just use it to amuse my son.

Here is a list of piano application designed for iPhone.

MiniPiano

Price: Free
Features: Only one octave available from C to C#
My comment:
Easy to play. You can do glissando by sliding your finger across the screen. But the cutoff tail is not smooth.

Chorddi-401

Price: Free
Features: Two octave available from F to E, saxophone sounds available.
My comment:
Hard to play because the key is too small on screen.

These are the only two iPhone piano apps that I installed on my iPhone.
Did you purchase other apps or install one? Please tell us how you feel about it in the comment.

Several other apps relating to piano are:

1. Pocket Piano ($0.99)
2. PianoWork (Free)
3. Easy Piano ($0.99)
4. RealPiano (Live Grand Piano)($1.99)
5. PockeStra Piano ($0.99)
6. Pocket Piano Plus ($2.99)
7. Pianoforce Remote (FREE)
8. Metronome - reloaded (Free)
9. Pianist ($3.99)
10. FingerPiano ($2.99)
11. Piano Man ($0.99)
12. A Thumb Piano ($0.99)
13. PianoNotes ($3.99)
14. PianoChordy ($3.99)
15. Pianochords ($1.99)
16. Piano2 ($0.99)
17. SAMsPiano ($0.99)