1) TonalEnergy ($3.99): Also known as, "TE Tuner".
Possibly the ultimate app for intonation practice, "TE Tuner" is not just a tuner. It plays drones (single notes or multiple/chords) and visually analyzes your intonation in real time. I've found a lot of references to playing to drone CDs on the SGF, but this takes it to another level. It includes a very flexible and configurable metronome and a tuner that seems to be just as good as ClearTune. It allows for custom temperaments and you can tune in the "analyzer" window which I actually prefer to ClearTune's standard tuner visuals. I thought the intonation analysis might be gimmicky but it tracks very well. For "fretless" instruments, it makes far more sense for metronome practice to include a reference drone to keep you in line. Another nice feature of TE Tuner is the ability to record a practice session so you can track your improvement. In the screenshot below, I'm playing a C major scale over a droned 'C' note at a slow speed to give myself a little time to adjust intonation by ear (not looking at the analysis screen for feedback). I'm not so good at it, but I think this might be the key to improving.
2) Steel Sidekick (FREE): I see a lot of people looking for online fretboard 'maps' but this one is tailored to steel. You can set up a custom copedent or use one of the presets. Pedals and knee levers are there to shift the notes. I use this regularly to find new ways to play chords and scales.
3) Jazz Harmony: Before getting this app, if someone asked me the notes of a chord, the 1-3-5 of a triad, or "what chords have E as the third?", I had to stop and count frets or scale degrees in my mind. There were only a few chords to which I could immediately give an answer, "G-B-D". This app changed all that in a matter of a couple of weeks. I would practice it 10-15 minutes a day on my iPhone while riding the London tube. The app is simple and effective. Choose the type of chord, number of notes, and it gives you a grid with a random note. In the screenshot below, the random note G is laid out and you have to fill in the rest. The major 1-3-5 triad is bottom to top and the minor triad is right to left. You can also choose which notes are in the pool of random selection so you can start with only a few or concentrate on weak spots.
4) AUM ($18.99): This is an excellent mixer app. I started using this because, when practicing with headphones, I wanted to use Bias FX for effects and amp modeling to play along to iReal Pro tracks. Well, iReal has no good way of adjusting it's volume vs other apps. AUM ties everything together, get the levels right and allows you to record practice sessions as well. You can feed apps into other apps to create signal chains to create a multi-fx unit/virtual pedal board. The AudioBus app also allows combining and routing of apps, but this is just simpler to use, in my opinion. It also includes a file player so you can, for example, record a dry steel line and then route it through effects chains to develop your sound without being near your instrument. The screenshot below is my practice preset. I have it set up with Bias for modeling, TE tuner for intonation/drone/metronome practice, AnyTune to slow down and study audio tracks, and iReal Pro for improvisation practice. I just unmute what I want to use and go at it. Well worth small amount of money to get everything working together conveniently.
5) Midi Designer Pro ($24.99): At some point, iPad modeling apps reached a point where the quality is as good or better than most dedicated hardware modelers. Compare Bias FX to your Pod and I think you'll be surprised. I have a Mustand III amp that allows two ways to amplify the iPad. 1) using the iPad as a multi-fx unit into the Mustang's modeling or 2) running the iPad into the Mustang's FX Loop return and using Bias amp modeling. I've gotten great results with both methods. Well, I started wanting more control over the effects and I don't really like Bias' guitar-centric layout. I've been experimenting with Midi Designer as a way to have a custom set of controls near my picking hand. Midi Designer is just what it sounds like. I've designed several layouts that allow me to do things that are not necesarily intended like a momentary stutter/killswitch or "boo-wah". IOS uses virtual midi, so I can either control parameters in Bias or in the AUM mixer. Muting tracks, panning, etc. In the screenshot below, I have the "channel morph" control that allows crossfading between two Bias signal chains, one clean and the other EQ'd for eBow with tape delay and a little dirt. The two buttons above the crossfader are set to send the crossfader slowly in one of the directions. For example, I could hit the "Ch 1" and go back to playing while the crossfader moves to the far left at my configured speed which I have set to 4 seconds. I could have also accomplished this by mapping the control to pan between two mixer channels in AUM, each with their own effects.
The killswitch is nice because a hardware killswitch pops unless you use distortion. This one has no such issue, is very responsive and I can make it as large or small as I want. It's a momentary switch that I have set up to control a a volume pedal at the beggining of my Bias effects chain. I could have also mapped it to the MUTE button on an AUM mixer channel. The X/Y pads on the right control the tape delay emulator and reverb.
6) Snippz (FREE): There are a lot of apps available to slow down and study audio. Anytune, Transcribe, etc. Snippz is the only one I could find for IOS that allows you to slow down a youtube video. Youtube content is tightly protected now so you can't grab a video and run it through one of the other slow-downer apps for legal reasons. Snippz probably hasn't been "caught" yet so if you need this, I'd download it before it's removed from the app store.
There are a lot of other apps that I use but they seem to be better known on the SGF than the ones I've described here. Anytune, iReal Pro, Chordbot, Session Band, ClearTune, Bias FX, etc. As well, there are simply too many exotic effects apps to mention. Convolution reverbs, multi-trackers, etc. I'm currently experimenting with Audulus 3 which is a modular synthesis designer for building custom effects from the ground up.
Hopefully, I've enlightened someone with all of this. IOS apps are priced extremely low considering their quality and I really don't see a need to ever go back to standalone pedals or multi-fx units.
If you want to have an ongoing discussion, please email me, don't use the Forum messaging which I detest! 
