iHandy Level App for iPhone
AutoCAD WS - Pull up drawings/plans,
Home Interior Layout Designer - Layouts and drawing for on the spot designing
Kravet/Lee Jofa - furniture and fabrics
Square - Run a credit card through your phone
iHandy Level - for an on the spot level to hang those pictures with
MacBox Units iPhone App - For the international designer, converts measurements + a built in ruler
Mark on Call - Created by an interior designer has tons of helpful features, including shopping lists, measurement tools, layout planners, etc
I.D. Wood - is a pocket guide to woods, including information about durability, sustainability, and color
Home Sizer - Calculates square footage and construction costs
3rd Whale - Helps you shop green.
Evernote - All-purpose organizational app, a great replacement for your giant design binders.
Craigsphone - Craigslist's mobile version for the iPhone
Now you can design like crazy from anywhere, anytime!