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.
It seems that the TM-50 is now the upgrade of the TM-40.
I also found the TMR-50 wich has a mic to record what you are playing and a USB port to transfert what was recorded to you PC. I have no idea if it is good or not, just thought it was interesting.
I have an android app called PitchLab. It was free and works better than my $100 Peterson Strobo, which never worked at all.
Then I got the $4 'Cleartune' app, but it won't respond to the low notes.
The Korg TM-40 costs more (unless you wait for it to be on sale... it can be from $26-71 compared to the CA-40 at $13-36), but also has a metronome, with sound/flashing LEDs, and a pendulum display. And you can use both the metronome and the tuner at once.