EasyRelease for model releases
GPS4CamPro to add location tags to your photos
LightTrac - this lets you plan out shots where the position of the sun or moon or shadows caused by either are important. Want the sun rising directly behind a building, dead center? It's only going to happen for a few days a year - this lets you map it out. It's also a handy way to look up moon phases and sunrise/sunset times. Sadly, cannot predict the weather months into the future, so if you're planning to travel for a specific shot on a specify day, here's hoping it doesn't rain
645 Pro - this is for using the phone as a camera rather than in conjunction with your Pentax, but it's handy. Gives you full manual control over shutter speed, aperture, ISO, white balance, and claims to save it as a high resolution tiff before any jpg compression is applied by the phone. I can't vouch one way or another, but I have a 16x20 print of the sun rising over our local farmer's market that looks pretty good, though I really wish I'd had my 15mm ltd and K5 with me that morning.
DOFMaster - Depth of Field calculator. You can just use their website, but it's handy to have it as an app sometimes.
LightMeter - Decent light meter/color temperature app. Add a
Luxi and your phone becomes a pretty decent incident light meter
ShutterSnitch - with an EyeFi or Flucard or any other wifi enabled camera, you can use this to preview your images apply rules to them (updating metadata and so on) and set off warnings for specific conditions -- for example, if you're going to be shooting something moving fast, you can set it to warn you if your shutter speed goes below some level (maybe I'm just clumsy, but I accidentally change my shutter speed and aperture settings all the time. Seems much worse with K3 than it was with K5, I think because the position of the dials on the grip is a bit different). Also ISO, aperture, focal length, and light level. It also zebra stripes blown out highlights and I think shadows as well. More useful on a tablet, since it gives you a bigger preview picture, but still handy. Also, if you're in a possibly dodgy situation where some jackbooted thug may demand that you erase your camera (or even if you're just worried about a card failure and want extra protection) you can configure it to upload your photos to an FTP server or lots of other online backup locations as soon as you take them.
DarkSky - to help find places where you can actually see some stars at night.
Square Register - to sell your work to people with credit cards
Seek Thermal - with the
Seek camera to check things like energy efficiency in your house (great justification for buying it!). If you do wildlife photos, it might help you spot something that you otherwise wouldn't see. FLIR One would work, too. It's just more expensive and more awkward to use. I had one and took it back after a few days.