This page lists some of my favorite apps, books, podcasts, and websites. If it's on this list, I've used it for my own practice.
tunable (tuner, metronome, recording)
earplugs for musicians:
Rosindust: Teaching, Learning and Life from a Cellist's Perspective (Cornelia Watkins)
The Art of Practicing: a Guide to Making Music from the Heart (Deline Bruser & Yehudi Menuhin)
websites & podcasts:
Neil Pasricha (international happiness guru) Blog