Update: Downloaded the Mood Track Diary (private) for iPhone because it was only $0.99 and I have a billion gift card money.

Looks like you can input your mood by the hour or by the day (whichever you choose). That might be good for identifying triggers (e.g., Maybe you felt "good" at 8am, but then at 11am you felt "pissed", so then you can identify what happened between 8am and 11am that made you pissed).

I might try lolagrace's suggestions to see which ones I like best.