Tuners
I notice David mention how some tuners are hard to work with and it resonated with me. i certainly have found a few tuners and apps which weren't really practical because they were so unstable.
so i thought i'd mention the Korg CA 40. JimiLee suggested this unit to me. i believe he recommends it to all his students, so its tried and true.
i found this to be a really practical little tuner when i was developing draw bend technique. i'd recommend it to anyone who feels the need for a visual reference in practice situations where HarpNinja (say) isnt practical.