I recently stumbled upon Geo Measure, an iPhone app which allows you to measure distances and land area on your phone.
The developer's app description:

A simple to use area and distance measurement tool for maps.
Have you ever wondered “How much acreage is that farm?” or “What is the distance between your house and the subway station?”. Are you curious to find out who has the most property in your neighborhood?

[...] This is a great tool for anyone who wants to find geographical areas or distances. Use the app for finding distances between far away places or just calculate the bike path before you actually attempt it.

[...] Locally save and email data as KML. These KML files are also available via iTunes.

Source: Apple App Store

So if you are in the field and want to know how far way a train stop is or how many acres a development site (roughly) contains, you can do it right on your phone.

I haven't used it yet, but am eager to try it. If you have, I'd enjoy hearing your thoughts!

Download this app: iPhone, iPad