I'm not afraid to admit that I'm slightly addicted to my iPhone. It's a really useful piece of tech, not just for communication, but for my work as well. Here's a list of apps I use almost daily for my motion design & post production work :
What The Font (Free, iTunes Link)
Take a picture of a sign, layout, billboard, or pretty much anything and upload it to What The Font. It operates much like the website and can be a lifesaver. Though it can be tricky to get matches back unless you have a 3G S, as the fixed focus camera on the original and 3G iPhones makes it tricky to get a decent picture.
cliqcliq Colors ($2.99, iTunes Link)
Choosing the right colors for a project is important and inspiration can come from anywhere at any time. Chose your own colors (up to 16) or use a photo as the basis for the palette. You can work in RGB, HSB, Gray or CMYK (for you print designers). When you're done you can name & rate your palette, or even send it in an email. The email is especially thorough, providing ACO, ASE, Office Open XML Color Theme, bitmap, plaintext, and CSV file formats, along with a preview.
ColorSlide (Free, iTunes Link)
Speaking of color palettes, you're probably familiar with Adobe Kuler. ColorSlide is basically an iPhone front-end allowing you to search, browse, and bookmark the palettes. Unfortunately, there's no way to sign in with your Kuler account, though you can email links.
Mill Colour (Free, iTunes Link)
A great little photo adjustment app by The Mill. Allows you to chose from predefined looks or adjust lift/gamma/gain (both luminance & separate RGB) and saturation on your own.
PS Mobile (Free, iTunes Link)
From Adobe themselves. Crop, adjust exposure/saturation/tint, apply filters and save & upload your completed image to photoshop.com. I admit I don't use this too often, but it's handy to have.
Animation Timer ($4.99, iTunes Link)
Tap out timing for your animation. It's similar to the "lap" feature on stopwatches, though instead of fractions of a second, you get frames, 35mm footage or timecode. It's handy for timing everything from edits to complex character animation.
EditCalc ($0.99, iTunes Link)
A simple timecode calculator. Works in 23.98, 24, 25, 29.97, 59.94, and 60 fps. You can also work in varying film footages, from 16mm 1perf all the way up to 70mm 5perf.
AJA DataCalc (Free, iTunes Link)
Recently released, this app will calculate storage for varying frame sizes/rates and codecs, and closely mirrors their Mac & Windows calculators in function. You can work in days/hours/min/sec or timecode. What I really like about the app is you can chose between working in KB (1000 bytes), KiB (1024 bytes) or even Bits, which is handy if you're working in Snow Leopard. One complaint is that they don't offer 720p24 as a preset, though you can use custom setups, so it's not that big of a deal. When you're all done you can mail a summary of your calculation; useful if you're on set and need to let your assistant know what's coming.
iBlueSky ($9.99, iTunes Link)
If you're not familiar with mind mapping, you might not care much about this app. But it's hands-down the best app for this purpose. What I really like is that I can email my maps as OPML files (along with other formats) and open it up with OmniOutliner on my Mac.
Pocket VFX ($0.99, iTunes Link)
This is just for fun. Framestore (vfx credits include Avatar, The Dark Knight, The Golden Compass, and Where the Wild Things Are) has released an app of their own. You, too can have Framestore's power in your own pocket!
Have other favorite design/post apps? Share them in the coments.