Correl shows me the link between what I do and how I feel. I can easily visualize which daily habits help (or hurt) my mood at the end of the day. Knowing which habit goals are strongly correlated to a positive mood is a powerful motivator.
Disclaimer: I made Correl.
It's simple, and it works. I've been writing code with Notepad++ every since it came out. One of the only products I've used for so long without trying something else - it's never given me a reason to switch.
I always prefer to get recommendations from people I know or respect - YourStack is a really efficient way to do that. And It replaces all of the time I used to spend searching "Best _____" lists and reviews. Seems so obvious now that someone has built it!
One of my favorite products ever...it's unlocked by the time I have it out of my pocket, and the pictures blow me away every time. I'm tempted to buy a few extra in case future versions aren't as good...